fix(fpga): include missing header file

Since transitioning over FEAT_SPE to the new feature checking scheme, we
make use of the new is_feat_spe_supported() function in the Arm FPGA
platform code. However this missed to include the header file, so the
build broke.

Add the arch_features.h header to make arm_fpga compile again.

Change-Id: I5c8feecfcc6fb5845a6671842850df1943086a58
Signed-off-by: Andre Przywara <andre.przywara@arm.com>