I guess commit 
15ea0e1e3e18 ("efi: Import certificates from UEFI Secure
Boot") attempted to add -fshort-wchar for building load_uefi.o, but it
has never worked as intended.
load_uefi.o is created in the platform_certs/ sub-directory. If you
really want to add -fshort-wchar, the correct code is:
  $(obj)/platform_certs/load_uefi.o: KBUILD_CFLAGS += -fshort-wchar
But, you do not need to fix it.
Commit 
8c97023cf051 ("Kbuild: use -fshort-wchar globally") had already
added -fshort-wchar globally. This code was unneeded in the first place.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
 
 integrity-$(CONFIG_LOAD_UEFI_KEYS) += platform_certs/efi_parser.o \
                                        platform_certs/load_uefi.o
 integrity-$(CONFIG_LOAD_IPL_KEYS) += platform_certs/load_ipl_s390.o
-$(obj)/load_uefi.o: KBUILD_CFLAGS += -fshort-wchar
 
 subdir-$(CONFIG_IMA)                   += ima
 obj-$(CONFIG_IMA)                      += ima/