util: include `stdarg.h`

util: include `stdarg.h`

Otherwise using uClibc compilation (OpenEmbedded [1] with `minimal-uclibc` for `MACHINE = "beagleboard"`) fails with the following error message.

| make[1]: Entering directory `/oe/build-minimal-uclibc/minimal-uclibc-dev/work/armv7a-oe-linux-uclibceabi/systemd-v26-r0/git'
|   CC     src/libsystemd_basic_la-util.lo
| src/util.c: In function 'acquire_terminal':
| src/util.c:2484:29: warning: cast increases required alignment of target type [-Wcast-align]
| src/util.c:2497:37: warning: cast increases required alignment of target type [-Wcast-align]
| src/util.c: In function 'rename_process':
| src/util.c:2905:17: warning: passing argument 1 of 'strncpy' discards qualifiers from pointer target type
| /oe/build-minimal-uclibc/minimal-uclibc-dev/sysroots/armv7a-oe-linux-uclibceabi/usr/include/string.h:87:14: note: expected 'char * restrict' but argument is of type 'const char *'
| src/util.c: In function 'cpu_set_malloc':
| src/util.c:3229:17: warning: implicit declaration of function 'CPU_ALLOC' [-Wimplicit-function-declaration]
| src/util.c:3229:25: warning: assignment makes pointer from integer without a cast
| src/util.c:3232:17: warning: implicit declaration of function 'CPU_ALLOC_SIZE' [-Wimplicit-function-declaration]
| src/util.c:3233:25: warning: implicit declaration of function 'CPU_ZERO_S' [-Wimplicit-function-declaration]
| src/util.c:3241:17: warning: implicit declaration of function 'CPU_FREE' [-Wimplicit-function-declaration]
|   CC     src/libsystemd_basic_la-label.lo
| In file included from src/label.c:27:0:
| src/util.h:358:41: error: expected declaration specifiers or '...' before 'va_list'
| make[1]: *** [src/libsystemd_basic_la-label.lo] Error 1
| make[1]: Leaving directory `/oe/build-minimal-uclibc/minimal-uclibc-dev/work/armv7a-oe-linux-uclibceabi/systemd-v26-r0/git'
| make: *** [all] Error 2
| + die 'oe_runmake failed'
| + oefatal 'oe_runmake failed'
| + echo FATAL: 'oe_runmake failed'
| FATAL: oe_runmake failed
| + exit 1
NOTE: package systemd-v26-r0: task do_compile: Failed
ERROR: Function 'do_compile' failed (see /oe/build-minimal-uclibc/minimal-uclibc-dev/work/armv7a-oe-linux-uclibceabi/systemd-v26-r0/temp/log.do_compile.4020 for further information)
ERROR: Task 13 (/oe/openembedded/recipes/systemd/systemd_git.bb, do_compile) failed with exit code '1'
ERROR: '/oe/openembedded/recipes/systemd/systemd_git.bb' failed

[1] http://www.openembedded.org/