The Mbed TLS importer accidentally imports Makefile and .gitignore
from Mbed TLS's `library/` directory. This commit restricts the
pattern to .h and .c files only, and removes the unnecessary files.
Files have been automatically imported by setting MBED_TLS_RELEASE to
mbedtls-2.24.0 in connectivity/mbedtls/tools/importer/Makefile and
running `make` in that directory.