build: fix sc_prohibit_readlink

I noticed this during 'make syntax-check':

prohibit_readlink
grep: Unmatched ( or \(

* cfg.mk (exclude_file_name_regexp--sc_prohibit_readlink): Fix
mismatched '('.

cfg.mk

@@ -801,7 +801,7 @@ exclude_file_name_regexp--sc_prohibit_raw_allocation = \
   ^(src/util/memory\.[ch]|examples/.*)$$
 
 exclude_file_name_regexp--sc_prohibit_readlink = \
-  ^(src/(util/util|lxc/lxc_container)\.c$$
+  ^src/(util/util|lxc/lxc_container)\.c$$
 
 exclude_file_name_regexp--sc_prohibit_setuid = ^src/util/util\.c$$