util: Add 'strstart()'.

ir/adt/util.h

@@ -69,4 +69,13 @@ static inline bool streq(char const *const a, char const *const b)
 	return strcmp(a, b) == 0;
 }
 
+static inline char const* strstart(char const* str, char const* start)
+{
+	do {
+		if (*start == '\0')
+			return str;
+	} while (*str++ == *start++);
+	return NULL;
+}
+
 #endif