build: enable code coverage

Enable code coverage for the project.  Valgrind not enabled in this
step.

Change-Id: I439d58b23d2bc29d6f602600319074c8fd8c04d6
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/.gitignore b/.gitignore
index dde04e4..2d173c1 100644
--- a/.gitignore
+++ b/.gitignore
@@ -10,6 +10,12 @@
 /test-driver
 /ylwrap
 
+# Code Coverage
+*.gcda
+*.gcno
+*.trs
+*-coverage*
+
 # http://www.gnu.org/software/autoconf
 
 /autom4te.cache