[QNX] Force UNIX-style separators on Windows hosts

The QNX toolchain, even on Windows, uses UNIX-style directory separators
('/').

Change-Id: I78575c9856b1700438c05ec0da81149688d85522
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>

mkspecs/common/qcc-base-qnx.conf

@@ -55,3 +55,8 @@ QMAKE_LINK              = $$QMAKE_CXX
 QMAKE_LINK_SHLIB        = $$QMAKE_CXX
 
 QMAKE_STRIPFLAGS_LIB    += --strip-unneeded
+
+equals(QMAKE_HOST.os, Windows) {
+    isEmpty(QMAKE_SH): error("This mkspec requires an MSYS environment.")
+    QMAKE_DIR_SEP       = /
+}