Make lrelease detection work on Windows.

Make sure to expand variable before testing.

Change-Id: I6b6f11782677c178fe3f2209f84887084940976a

rbutil/rbutilqt/rbutilqt.pro

@@ -111,11 +111,11 @@ win32:!cross {
 }
 lrelease.commands = $$LRELEASE -silent $$_PRO_FILE_
 QMAKE_EXTRA_TARGETS += lrelease
-exists(LRELEASE) {
+exists($$LRELEASE) {
     message("using lrelease at $$LRELEASE")
     PRE_TARGETDEPS += lrelease
 }
-!exists(LRELEASE) {
+!exists($$LRELEASE) {
     warning("could not find lrelease. Skipping translations.")
 }