hexagon: vdso: include asm/elf.h for arch_setup_additional_pages() prototype

Clang warns:

  arch/hexagon/kernel/vdso.c:49:5: warning: no previous prototype for function 'arch_setup_additional_pages' [-Wmissing-prototypes]
     49 | int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
        |     ^
  arch/hexagon/kernel/vdso.c:49:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
     49 | int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
        | ^
        | static
  1 warning generated.

Include the header that declares the prototype to clear up the warning.

Link: https://lkml.kernel.org/r/20231130-hexagon-missing-prototypes-v1-15-5c34714afe9e@kernel.org
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Brian Cain <bcain@quicinc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>