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/check_hostent.c

@@ -20,6 +20,7 @@
 
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include <support/check.h>
 #include <support/format_nss.h>
 #include <support/run_diff.h>