fix: select packages by C example file rather than binding.gyp

`examples-random-c` was using `PACKAGES_PATTERN='binding.gyp'` to find
candidate packages, but NAPI utility packages (e.g. `@stdlib/napi/argv-*`)
have `binding.gyp` for their Node.js add-on yet ship no `examples/c/*.c`
file. Selecting one of those packages caused `examples-c` to fail, which
made the `random_examples` CI workflow fail on ~53% of runs (16/30).

Changing to `PACKAGES_PATTERN='example.c'` targets the 1538 packages that
actually have a compilable C example in their `examples/c/` directory,
eliminating the false-positive package selection entirely.

Author: claude

tools/make/lib/examples/c.mk

@@ -113,7 +113,7 @@ examples-c-files:
 # make examples-random-c RANDOM_SELECTION_SIZE=10
 #/
 examples-random-c: $(NODE_MODULES)
-	$(QUIET) $(MAKE) -f $(this_file) -s list-random-lib-pkgs PACKAGES_PATTERN='binding.gyp' | while read -r pkg; do \
+	$(QUIET) $(MAKE) -f $(this_file) -s list-random-lib-pkgs PACKAGES_PATTERN='example.c' | while read -r pkg; do \
 		echo ""; \
 		echo "Running example: $$pkg"; \
 		NODE_ENV="$(NODE_ENV_EXAMPLES)" \