unit-test: Fix PEP8 W605 violation

W605: invalid escape sequence '\.'

Change-Id: I8cb5d152b041fef0f9be72d7104b56bd180f134c
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/scripts/unit-test.py b/scripts/unit-test.py
index 8224fb3..5024197 100755
--- a/scripts/unit-test.py
+++ b/scripts/unit-test.py
@@ -360,7 +360,7 @@
 
 
 def run_cppcheck():
-    match_re = re.compile('((?!\.mako\.).)*\.[ch](?:pp)?$', re.I)
+    match_re = re.compile(r'((?!\.mako\.).)*\.[ch](?:pp)?$', re.I)
     cppcheck_files = []
     stdout = subprocess.check_output(['git', 'ls-files'])