powerpc/pseries/htmdump: Include header file to get is_kvm_guest() definition

powerpc/pseries/htmdump: Include header file to get is_kvm_guest() definition

htmdump_init calls is_kvm_guest() to check for guest environment.
is_kvm_guest() is defined in kvm_guest.h header file. Without including
the header file, build hits error failing to find the function.

  arch/powerpc/platforms/pseries/htmdump.c: In function 'htmdump_init':
  arch/powerpc/platforms/pseries/htmdump.c:469:6: error: implicit declaration of function 'is_kvm_guest';
  did you mean '__key_get'? [-Werror=implicit-function-declaration]
  if (is_kvm_guest()) {
      ^~~~~~~~~~~~
      __key_get

This is observed in configs where CONFIG_KVM_GUEST is disabled.
Include header file explicitly to avoid the build error

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202505061324.elUl4njU-lkp@intel.com/
Signed-off-by: Athira Rajeev <atrajeev@linux.ibm.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250506135232.69014-1-atrajeev@linux.ibm.com