suppress error output from pkg-config

under normal circumstances, any errors will be noticed already by the
pkg-config --exists call, which is silent anyway. therefore this doesn't
change anything in normal qmake usage.

however, lupdate's and creator's evaluators skip the --exists calls and
subsequently invoke the normal query, which then prints useless noise to
the terminal.

Task-number: QTBUG-28159
Change-Id: I536412060f3830aafeb0587f855cd6af11227bca
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>

mkspecs/features/qt_functions.prf

@@ -304,6 +304,11 @@ defineReplace(pkgConfigExecutable) {
         }
     }
 
+    equals(QMAKE_HOST.os, Windows): \
+        PKG_CONFIG += 2> NUL
+    else: \
+        PKG_CONFIG += 2> /dev/null
+
     return($$PKG_CONFIG)
 }