.gitignore: Add default.profraw

Emitted in the CWD when you run executables with the coverage enabled.
I find myself running the unittests from the project root a lot, and this file is annoying to skip when staging code.

Change-Id: I25557c941b2723aa300fcde827d281b054cbf2fd
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/42843
Reviewed-by: James Price <jrprice@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
diff --git a/.gitignore b/.gitignore
index 3805ec5..17e10dd 100644
--- a/.gitignore
+++ b/.gitignore
@@ -9,6 +9,7 @@
 buildtools
 cmake-build-*/
 coverage.summary
+default.profraw
 lcov.info
 out
 testing