Makefile: Remove BUILD_DIR from qapi-dir

qapi-dir does not need an absolute path. All other build directories
are relative. When BUILD_DIR is removed, the build output looks better
(no long lines with absolute paths when everything else uses short
lines):

  GEN   qapi-generated/qga-qapi-types.c
  CC    qapi-generated/qga-qapi-types.o
  GEN   qapi-generated/qga-qapi-visit.c
  CC    qapi-generated/qga-qapi-visit.o
  GEN   qapi-generated/qga-qmp-marshal.c
  CC    qapi-generated/qga-qmp-marshal.o

Using a relative path also avoids potential problems when BUILD_DIR
includes blanks.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

Makefile

@@ -171,7 +171,7 @@ fsdev/virtfs-proxy-helper$(EXESUF): LIBS += -lcap
 qemu-img-cmds.h: $(SRC_PATH)/qemu-img-cmds.hx
 	$(call quiet-command,sh $(SRC_PATH)/scripts/hxtool -h < $< > $@,"  GEN   $@")
 
-qapi-dir := $(BUILD_DIR)/qapi-generated
+qapi-dir := qapi-generated
 qemu-ga$(EXESUF): LIBS = $(LIBS_QGA)
 qemu-ga$(EXESUF): QEMU_CFLAGS += -I $(qapi-dir)