efi_loader: make get_load_options() static

In program initrddump.efi function get_load_options() can be static.

This avoids a warning when building with 'make W=1':

    lib/efi_loader/initrddump.c:442:6: warning:
    no previous prototype for ‘get_load_options’ [-Wmissing-prototypes]
      442 | u16 *get_load_options(void)
          |      ^~~~~~~~~~~~~~~~

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>