Cocoa QPA: Remove usage of OBJECTIVE_SOURCES

Change-Id: I5924ab0ddb442624f5aeeef023428be228348707
Reviewed-by: Jake Petroules <jake.petroules@qt.io>

src/plugins/platforms/cocoa/cocoa.pro

@@ -1,6 +1,6 @@
 TARGET = qcocoa
 
-OBJECTIVE_SOURCES += main.mm \
+SOURCES += main.mm \
     qcocoaintegration.mm \
     qcocoatheme.mm \
     qcocoabackingstore.mm \
@@ -30,9 +30,8 @@ OBJECTIVE_SOURCES += main.mm \
     qcocoasystemtrayicon.mm \
     qcocoaintrospection.mm \
     qcocoakeymapper.mm \
-    qcocoamimetypes.mm
-
-SOURCES += messages.cpp
+    qcocoamimetypes.mm \
+    messages.cpp
 
 HEADERS += qcocoaintegration.h \
     qcocoatheme.h \
@@ -66,7 +65,7 @@ HEADERS += qcocoaintegration.h \
     qcocoamimetypes.h
 
 qtConfig(opengl.*) {
-    OBJECTIVE_SOURCES += qcocoaglcontext.mm
+    SOURCES += qcocoaglcontext.mm
 
     HEADERS += qcocoaglcontext.h
 }
@@ -85,7 +84,7 @@ CONFIG += no_app_extension_api_only
 qtHaveModule(widgets) {
     QT_FOR_CONFIG += widgets
 
-    OBJECTIVE_SOURCES += \
+    SOURCES += \
         qpaintengine_mac.mm \
         qprintengine_mac.mm \
         qcocoaprintersupport.mm \