Make configure fail if a52 headers are not found

configure.ac

@@ -3088,7 +3088,9 @@ then
         VLC_ADD_CPPFLAGS([a52tofloat32],[${CPPFLAGS_test}])
       ],[
         AC_MSG_ERROR([Could not find liba52 on your system: you may get it from http://liba52.sf.net/. Alternatively you can use --disable-a52 to disable the a52 plugin.])
-      ])
+        ]),
+    ],[
+      AC_MSG_ERROR([Could not find liba52 on your system: you may get it from http://liba52.sf.net/. Alternatively you can use --disable-a52 to disable the a52 plugin.])
     ])
     CPPFLAGS="${CPPFLAGS_save}"
     LDFLAGS="${LDFLAGS_save}"