Fix Qt5 libs linking to Qt5WebEngineCore.dll for Windows

MSVC linker needs file extension, otherwise it searches for .obj
files, but Qt has Qt5*.lib files.

Change-Id: I4fd5e1d6001ea309da456c43ffc79cb5944a1ef3
Reviewed-by: Andras Becsi <andras.becsi@digia.com>
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>

tools/qmake/mkspecs/features/gyp_generator.prf

@@ -94,7 +94,8 @@ GYP_CONTENTS += "    ],"
 }
 GYP_CONTENTS += "    'link_settings': {" \
                 "      'libraries': ["
-for (library, LIBRARIES): GYP_CONTENTS += "        '$$library',"
+win32: for (library, LIBRARIES): GYP_CONTENTS += "        '$${library}.lib',"
+else:  for (library, LIBRARIES): GYP_CONTENTS += "        '$$library',"
 macx {
     FRAMEWORKS = $$find(LIBS, "Q*")
     FRAMEWORKS = $$unique(FRAMEWORKS)