powerpc/64e: Fix kexec build error

When building ppc64_book3e_allmodconfig the build fails with:

  arch/powerpc/kexec/file_load_64.c:1063:14: error: implicit declaration of function ‘firmware_has_feature’
   1063 |         if (!firmware_has_feature(FW_FEATURE_LPAR))
        |              ^~~~~~~~~~~~~~~~~~~~

Add a direct include of asm/firmware.h to fix the error.

Fixes: b1fc44ea

 ("pseries/iommu/ddw: Fix kdump to work in absence of ibm,dma-window")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220803063152.1249270-1-mpe@ellerman.id.au