gf-server.cabal: make hsql-mysql optional like fastcgi

content-service depends on hsql-mysql, which depends on a C library, so like
fastcgi, only compile it if you ask for it explicitly with -ffastcgi.

src/server/gf-server.cabal

@@ -68,11 +68,11 @@ executable pgf-service
 
 executable content-service
   if flag(fastcgi)
-    build-depends: fastcgi >= 3001.0.2.2
+    build-depends: fastcgi >= 3001.0.2.2, hsql-mysql
     buildable: True
   else
     buildable: False
 
   build-depends: base >=4.2 && <5,
-                 hsql, hsql-mysql, old-locale
+                 hsql, old-locale
   main-is: ContentService.hs