.gitignore: ignore qa-assets/ folder

This commit ignores the qa-assets/ folder in case a user is running
the cov_fuzz target without DIR_FUZZ_SEED_CORPUS set. In this case,
the qa-assets folder is assumed to live in the bitcoin directory and
should be ignored by git.

.gitignore

@@ -122,6 +122,7 @@ total.coverage/
 fuzz.coverage/
 coverage_percent.txt
 /cov_tool_wrapper.sh
+qa-assets/
 
 #build tests
 linux-coverage-build