add a basic .gitignore

with a simple build system where objects are placed along the source,
'git status' annoyingly reports after build:

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
  (commit or discard the untracked or modified content in submodules)
	modified:   deps/libchdr (untracked content)

It is possible to workaround this but it's inconvenient, much simpler if
libchdr itself comes with a .gitignore file.

.gitignore

@@ -0,0 +1,3 @@
+*.o
+*.d
+build/