va: include <unistd.h> for getuid/getgid in secure_getenv fallback

On platforms like NetBSD (and other non-glibc systems), getuid(),
geteuid(), getgid(), and getegid() are not implicitly available.
They require an explicit include of <unistd.h> per POSIX. Without
it the build fails with undefined function errors.

Fixes: #888
Signed-off-by: Carl.Zhang <carl.zhang@intel.com>

Author: XinfengZhang

va/va_internal.h

@@ -34,6 +34,7 @@ extern "C" {
 /* No setuid/setgid on Windows, secure_getenv is just getenv */
 #define secure_getenv getenv
 #else
+#include <unistd.h>
 static inline char * secure_getenv(const char *name)
 {
     if (getuid() == geteuid() && getgid() == getegid())