boot/arm-trusted-firmware: fix fiptool linking issue on hosts without openssl

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/5834758777

Commit e7d16c35a (boot/arm-trusted-firmware: fix the RPATH of fiptool) tried
to fix the build of host-fiptool, but forgot to pass HOST_CFLAGS.

On hosts without (compatible) openssl development headers, this breaks
the build when it cannot find the openssl headers:

fiptool_platform.h:19:11: fatal error: openssl/sha.h: No such file or directory

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit e6ef64d955f34db773addad52c7af17e111d7f13)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/arm-trusted-firmware/arm-trusted-firmware.mk

@@ -64,7 +64,7 @@ ARM_TRUSTED_FIRMWARE_MAKE_OPTS += \
 	$(call qstrip,$(BR2_TARGET_ARM_TRUSTED_FIRMWARE_ADDITIONAL_VARIABLES)) \
 	PLAT=$(ARM_TRUSTED_FIRMWARE_PLATFORM) \
 	TARGET_BOARD=$(ARM_TRUSTED_FIRMWARE_TARGET_BOARD) \
-	HOSTCC="$(HOSTCC) $(HOST_LDFLAGS)"
+	HOSTCC="$(HOSTCC) $(HOST_CFLAGS) $(HOST_LDFLAGS)"
 
 ifeq ($(BR2_TARGET_ARM_TRUSTED_FIRMWARE_SSP),y)
 ARM_TRUSTED_FIRMWARE_MAKE_OPTS += \