gst/gstinfo.h: Add an explicit variable importation needed on VS6 (only for MSC_VER)

gst/gstinfo.h: Add an explicit variable importation needed on VS6 (only for MSC_VER)

Original commit message from CVS:
* gst/gstinfo.h:
Add an explicit variable importation needed on VS6 (only for MSC_VER)
Define M_PI which is used in files which are including gstinfo.h.
VS6 includes doesn't define it.
* win32/common/libgstbase.def:
* win32/common/libgstcontroller.def:
* win32/common/libgstreamer.def:
Add new exported functions and variables.
* win32/vs6/libgstcontroller.dsp:
* win32/vs6/libgstreamer.dsp:
Update the list of files to build.