trusted-firmware-a.mk: make sure include directory exists

trusted-firmware-a.mk: make sure include directory exists

ARM Trusted Firmware builds do not depend on any target libraries as
they are bare-metal builds. However, the compiler aborts due to
-Werror=missing-include-dirs if the include dir doesn't exists and this
can happen when building with parallelisation as that makes it likely
for arm-trusted-firmware-* to be build very early before any of the
libraries which would implicitely create the directory.
Fix this by making sure the include dir exists before building.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>