tests/tcg: add .gitignore for in source builds

This hides the new build artefacts from the re-organised TCG tests when
you are doing an in-source build.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>

tests/tcg/.gitignore

+# These are build artefacts which only appear when you are doing
+# builds directly in the source tree.
+config-*.mak
+*-softmmu/
+*-linux-user/