configure: fix $$qtConfFindInPath() on windows

uses of this function (or the "files" stanza in configure.json) which
don't explicitly target windows don't specify the .exe extension, so we
need to add it automatically if it's missing.

Task-number: QTBUG-57436
Change-Id: I1994378399bc3466c32ee065e752516f42652975
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

mkspecs/features/qt_configure.prf

@@ -378,6 +378,7 @@ defineReplace(qtConfFindInPathList) {
 
 defineReplace(qtConfFindInPath) {
     ensurePathEnv()
+    equals(QMAKE_HOST.os, Windows):!contains(1, .*\\.exe): 1 = $${1}.exe
     return($$qtConfFindInPathList($$1, $$2 $$QMAKE_PATH_ENV))
 }