Kernel/LibC: Implement mprotect

There may be some race conditions with this but i think this is good
enough to start with

userspace/libraries/LibC/include/sys/syscall.h

@@ -52,6 +52,7 @@ __BEGIN_DECLS
 	O(SYS_SYNC,				sync)			\
 	O(SYS_MMAP,				mmap)			\
 	O(SYS_MUNMAP,			munmap) 		\
+	O(SYS_MPROTECT,			mprotect) 		\
 	O(SYS_TTY_CTRL,			tty_ctrl)		\
 	O(SYS_POWEROFF,			poweroff)		\
 	O(SYS_FCHMODAT,			fchmodat)		\

userspace/libraries/LibC/sys/mman.cpp

@@ -24,6 +24,11 @@ int munmap(void* addr, size_t len)
 	return syscall(SYS_MUNMAP, addr, len);
 }
 
+int mprotect(void* addr, size_t len, int prot)
+{
+	return syscall(SYS_MPROTECT, addr, len, prot);
+}
+
 int msync(void* addr, size_t len, int flags)
 {
 	pthread_testcancel();
@@ -45,8 +50,3 @@ int mlock(const void*, size_t)
 {
 	ASSERT_NOT_REACHED();
 }
-
-int mprotect(void*, size_t, int)
-{
-	ASSERT_NOT_REACHED();
-}