efi_loader: initrddump: Actually use the custom CFLAGS

It seems a copy'n'paste typo when tool had been introduced.
It has never had the 'exit' suffix in the file name. Hence,
the custom CFLAGS never been applied and, for example, BFD
linker complains:

  LD      lib/efi_loader/initrddump_efi.so
  ld.bfd: lib/efi_loader/initrddump.o: warning: relocation in read-only section `.text.efi_main'
  ld.bfd: warning: creating DT_TEXTREL in a shared object

Remove wrong 'exit' suffix from the custom CFLAGS variable.

Fixes: 65ab48d6 ("efi_selftest: provide initrddump test tool")
Fixes: 9c045a49 ("efi_loader: move dtbdump.c, initrddump.c to lib/efi_loader")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>