Include sys/types.h header to get mode_t

Do not depend on it being included by other system headers.
It is not included by others on musl-libc, for example.

Source/cmStandardIncludes.h

@@ -54,6 +54,8 @@
 
 #if defined( _MSC_VER )
 typedef unsigned short mode_t;
+#else
+# include <sys/types.h>
 #endif
 
 // use this class to shrink the size of symbols in .o files