cross-build: string.h uses mode_t, so we need sys/types.h

string.h defines strmode with a mode_t argument. POSIX states that one
must include sys/types.h to get mode_t, so do that here. This makes musl
happier. We know that sys/types.h will include sys/cdefs.h, so just
replace the latter with the former.

Co-authored-by:		Val Packett <val@packett.cool>
Sponsored by:		Netflix
Pull Request:		https://github.com/freebsd/freebsd-src/pull/1066
Reviewed by:		val_packett.cool
Differential Revision:	https://reviews.freebsd.org/D45353

tools/build/cross-build/include/linux/string.h

@@ -44,7 +44,7 @@
  */
 #include <stdlib.h>
 
-#include <sys/cdefs.h>
+#include <sys/types.h>
 
 __BEGIN_DECLS
 #if !defined(__GLIBC__) || \