BaseTools: update svn url in building-gcc.txt

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2-buildtools/code/trunk/BaseTools@2583 c2973edb-eda0-4c78-bc6a-9341b269661f