Use malloc for prism string source

Prism will later free this string via free rather than xfree, so we need
to use malloc rather than xmalloc.

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>
Co-authored-by: Matthew Draper <matthew@trebex.net>

prism_compile.c

@@ -10840,7 +10840,7 @@ pm_read_file(pm_string_t *string, const char *filepath)
         }
 
         size_t length = (size_t) len;
-        uint8_t *source = xmalloc(length);
+        uint8_t *source = malloc(length);
         memcpy(source, RSTRING_PTR(contents), length);
         *string = (pm_string_t) { .type = PM_STRING_OWNED, .source = source, .length = length };