[BUILD] 'make tags' did not consider files ending in '.c'

A missing parenthesis made the output of find apply only to files
'.h' making the tags useless.
diff --git a/Makefile b/Makefile
index 4c13355..1071c85 100644
--- a/Makefile
+++ b/Makefile
@@ -543,7 +543,7 @@
 	rm -f haproxy-$(VERSION) nohup.out gmon.out
 
 tags:
-	find src include -name '*.c' -o -name '*.h' -print0 | \
+	find src include \( -name '*.c' -o -name '*.h' \) -print0 | \
 	   xargs -0 etags --declarations --members
 
 tar:	clean