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>
diff --git a/plat/arm/board/arm_fpga/fpga_bl31_setup.c b/plat/arm/board/arm_fpga/fpga_bl31_setup.c
index 5cc1e76..1a38f25 100644
--- a/plat/arm/board/arm_fpga/fpga_bl31_setup.c
+++ b/plat/arm/board/arm_fpga/fpga_bl31_setup.c
@@ -7,6 +7,7 @@
 #include <assert.h>
 #include <errno.h>
 
+#include <arch_features.h>
 #include <common/fdt_fixup.h>
 #include <common/fdt_wrappers.h>
 #include <drivers/arm/gicv3.h>