Corrected GST_SDK set statement

GSTREAMER environment variable set wrong for 64 installation.

Change set statement on line 40 from 
set GST_SDK=%GSTREAMER_SDK_ROOT_X64%
to 
set GST_SDK=%GSTREAMER_SDK_ROOT_X86_64%
Merge pull request #13 from gannet/patch-1

Corrected GST_SDK set statement