bultin: mark unsafe functions

vlib/builtin/builtin.v

@@ -153,6 +153,7 @@ pub fn print(s string) {
 
 __global total_m i64 = 0
 //__global nr_mallocs int = 0
+[unsafe_fn]
 pub fn malloc(n int) byteptr {
 	if n < 0 {
 		panic('malloc(<0)')
@@ -185,6 +186,7 @@ pub fn calloc(n int) byteptr {
 	return C.calloc(n, 1)
 }
 
+[unsafe_fn]
 pub fn free(ptr voidptr) {
 	C.free(ptr)
 }

vlib/strings/builder_c.v

@@ -39,5 +39,5 @@ pub fn (b mut Builder) cut(n int) {
 }
 
 pub fn (b mut Builder) free() {
-	free(b.buf.data)
+	//free(b.buf.data)
 }