kbuild: remove useless $(gen) variable in Makefile.headersinst

We have no true case for the $(if $(gen), ...) conditional.  Drop it
to simplify the gendir calculation.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/Makefile.headersinst

@@ -39,9 +39,6 @@ skip-inst := $(if $(filter %/uapi,$(obj)),1)
 
 ifeq ($(skip-inst),)
 
-# generated header directory
-gen := $(if $(gen),$(gen),$(subst include/,include/generated/,$(obj)))
-
 # Kbuild file is optional
 kbuild-file := $(srctree)/$(obj)/Kbuild
 -include $(kbuild-file)
@@ -53,7 +50,7 @@ endif
 
 installdir    := $(INSTALL_HDR_PATH)/$(subst uapi/,,$(_dst))
 
-gendir        := $(objtree)/$(gen)
+gendir        := $(objtree)/$(subst include/,include/generated/,$(obj))
 header-files  := $(notdir $(wildcard $(srcdir)/*.h))
 header-files  += $(notdir $(wildcard $(srcdir)/*.agh))
 header-files  := $(filter-out $(no-export-headers), $(header-files))