gitignore: remove unnecessary entries, add subprojects/

The previous entries aren't used because the build files are
self-contained in the Meson build directory.

Add subprojects/ because it's common for developers to override
Meson dependencies via subproject symlinks.

.gitignore

@@ -1,9 +1,3 @@
-.clang_complete
-*.o
-*.a
-bin/
-test/
-build/
-build-*/
-wayland-*-protocol.*
-wlr-example.ini
+/build/
+/build-*/
+/subprojects/