allow running msvc2013 mkspecs from msvc2015 shell

Visual Studio 2015 integrates the Windows Runtime development
tools, including the msvc2013 compilers.
This patch fixes the error that mspdb120.dll is missing.
It is typically included through PATH variable of the shell,
which points to the "wrong" location in this case.

Change-Id: I46289721912d6b517c6083612582f67536d28b11
Reviewed-by: Andrew Knight <andrew.knight@intopalo.com>

qmake/generators/win32/msvc_nmake.cpp

@@ -231,6 +231,9 @@ NmakeMakefileGenerator::writeMakefile(QTextStream &t)
                             << kitDir + QStringLiteral("/include/shared")
                             << kitDir + QStringLiteral("/include/winrt");
                 }
+
+                binDirs << vcInstallDir + QStringLiteral("/bin");
+
                 // Inherit PATH
                 binDirs << QString::fromLocal8Bit(qgetenv("PATH")).split(QLatin1Char(';'));