Fix redefinition of _GNU_SOURCE.

libc/misc/getloadavg/getloadavg.c:26: warning: "_GNU_SOURCE" redefined
   26 | #define _GNU_SOURCE
      |
In file included from <command-line>:
./include/libc-symbols.h:52: note: this is the location of the previous definition
   52 | #define _GNU_SOURCE     1

Signed-off-by: Dmitry Chestnykh <dm.chestnykh@gmail.com>

libc/misc/getloadavg/getloadavg.c

@@ -23,7 +23,9 @@
  * Imported from musl C library
  */
 
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif /* _GNU_SOURCE */
 #include <stdlib.h>
 #include <sys/sysinfo.h>