os-wrappers-test: Handle fcntl() being declared as a macro

On some systems (e.g. FreeBSD with the latest epoll-shim), fcntl is
declared as a macro instead of a function. Wrap the definition here in
parantheses to avoid function-macro expansion.

Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>

tests/os-wrappers-test.c

@@ -100,7 +100,7 @@ socket(int domain, int type, int protocol)
 }
 
 __attribute__ ((visibility("default"))) int
-fcntl(int fd, int cmd, ...)
+(fcntl)(int fd, int cmd, ...)
 {
 	va_list ap;
 	int arg;