NASM: trailing slash for ASFLAGS includes

Fix out-of-tree builds using NASM. NASM expects its include paths to
have a trailing slash. These aren't used used when doing in-tree builds
(./configure)

Change-Id: I38d469d15acb1b7e65733a2e5ca8c9d86fa4ad86

build/make/Makefile

@@ -65,7 +65,7 @@ endif
 BUILD_ROOT?=.
 VPATH=$(SRC_PATH_BARE)
 CFLAGS+=-I$(BUILD_PFX)$(BUILD_ROOT) -I$(SRC_PATH)
-ASFLAGS+=-I$(BUILD_PFX)$(BUILD_ROOT) -I$(SRC_PATH)
+ASFLAGS+=-I$(BUILD_PFX)$(BUILD_ROOT)/ -I$(SRC_PATH)/
 DIST_DIR?=dist
 HOSTCC?=gcc
 TGT_ISA:=$(word 1, $(subst -, ,$(TOOLCHAIN)))