Fix Python escape character warning

[maciejopalinski]

/home/matthew/Projects/LiteX-CNC/src/litexcnc/cli/install_driver.py:137: SyntaxWarning: "\." is an invalid escape sequence. Such sequences will not work in the future. Did you mean "\\."? A raw string is also an option.
  if re.search("litexcnc_.*\.c", file.name):
/home/matthew/Projects/LiteX-CNC/src/litexcnc/cli/install_driver.py:149: SyntaxWarning: "\." is an invalid escape sequence. Such sequences will not work in the future. Did you mean "\\."? A raw string is also an option.
  if re.search("litexcnc_.*\.c", file.name):

this PR fixes the above warning by using raw string