Change wildcard . to literal . in regex for copying python.

On Windows we copy python27(_d).dll to the bin directory.  We do
this by looking at the PYTHON_LIBRARY specified by the user, which
is something like C:\foo\python27_d.lib, and replacing ".lib" with
".dll".  But ".lib" as a regex will also match "flib", etc.  So
make this a literal . instead of a wildcard .

llvm-svn: 226858