runtime/cgo: add cast in C code to avoid C compiler warning

Fixes #44340

Change-Id: Id80dd1f44a988b653933732afcc8e49a826affc4
Reviewed-on: https://go-review.googlesource.com/c/go/+/293209
Reviewed-by: Andrew G. Morgan <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>
Trust: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>

Author: ianlancetaylor

src/runtime/cgo/linux_syscall.c

@@ -32,7 +32,7 @@ typedef struct {
 
 #define SET_RETVAL(fn) \
   uintptr_t ret = (uintptr_t) fn ; \
-  if (ret == -1) {                 \
+  if (ret == (uintptr_t) -1) {	   \
     x->retval = (uintptr_t) errno; \
   } else                           \
     x->retval = ret