support: Simplify compiling most of support/ outside of glibc

Some include files were missing because they are implied by the
in-tree build process.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

support/support_format_hostent.c

@@ -21,6 +21,7 @@
 #include <arpa/inet.h>
 #include <errno.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <support/support.h>
 #include <support/xmemstream.h>