rpmsg: retarget: use stdint.h instead of sys/types.h

sys/types.h is not available in some compiler, uses stdint.h
for uintX_t or intX_t types.

Signed-off-by: Wendy Liang <jliang@xilinx.com>

lib/include/openamp/rpmsg_retarget.h

@@ -3,7 +3,7 @@
 
 #include <metal/mutex.h>
 #include <openamp/open_amp.h>
-#include <sys/types.h>
+#include <stdint.h>
 
 #if defined __cplusplus
 extern "C" {