fix QMAKE_DEFAULT_INCDIRS detection with Xcode targeting uikit platforms

uikit/sdk.prf replaces QMAKE_MAC_SDK_PATH with a make expansion of that
variable, which of course does not work when we use the contents
directly.

amends 6d5489f5d.

Task-number: QTBUG-61690
Change-Id: Id77dff8ee7d737dd35f74cc7d39faaa50b4b1ab9
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

mkspecs/features/toolchain.prf

@@ -40,7 +40,12 @@ isEmpty($${target_prefix}.INCDIRS) {
         # While technically incorrect but without any likely practical effect,
         # UIKit simulator platforms will see the device SDK's sysroot in
         # QMAKE_DEFAULT_*DIRS, because they're handled in a single build pass.
-        darwin: cxx_flags += -isysroot $$QMAKE_MAC_SDK_PATH
+        darwin {
+            uikit:macx-xcode: \
+                cxx_flags += -isysroot $$sdk_path_device.value
+            else: \
+                cxx_flags += -isysroot $$QMAKE_MAC_SDK_PATH
+        }
 
         rim_qcc: \
             # Need the cc1plus and ld command lines to pick up the paths