Fixing quotation mark usage for Windows

The Windows command shell try to interpret the single quotation mark
as a command.

Change-Id: I4dc4382bdbe0c90565f6ac56c33a80971669344c
Reviewed-by: Andras Becsi <andras.becsi@digia.com>
Reviewed-by: Michael Bruning <michael.bruning@digia.com>

tools/qmake/mkspecs/features/functions.prf

@@ -10,8 +10,8 @@ defineTest(isPlatformSupported) {
 }
 
 defineTest(isPythonVersionSupported) {
-  python_major_version = $$system("python -c 'import sys; print sys.version_info.major'")
-  python_minor_version = $$system("python -c 'import sys; print sys.version_info.minor'")
+  python_major_version = $$system('python -c "import sys; print sys.version_info.major"')
+  python_minor_version = $$system('python -c "import sys; print sys.version_info.minor"')
   greaterThan(python_major_version, 1): greaterThan(python_minor_version, 6): return(true)
   return(false)
 }