libroot: define restrict for GCC < 4.

Should fix GCC 2 build.

src/system/libroot/posix/musl/string/strcat.c

@@ -1,5 +1,9 @@
 #include <string.h>
 
+#if __GNUC__ < 4
+#define restrict
+#endif
+
 char *strcat(char *restrict dest, const char *restrict src)
 {
 	strcpy(dest + strlen(dest), src);

src/system/libroot/posix/musl/string/strncat.c

@@ -1,5 +1,9 @@
 #include <string.h>
 
+#if __GNUC__ < 4
+#define restrict
+#endif
+
 char *strncat(char *restrict d, const char *restrict s, size_t n)
 {
 	char *a = d;