cmake: Fix inconsistency regarding mbed TLS include directory

Previously, one had to set MBEDTLS_INCLUDE_DIR to make CMake find the
headers, but the system complained that mbed TLS wasn't found due to
MBEDTLS_INCLUDE_DIRS (note the trailing s) was not set. This commit
attempts to fix that.

Closes https://github.com/curl/curl/pull/1541