powerpc/boot: Fix build with gcc 15

Similar to x86 the ppc boot code does not build with GCC 15.

Copy the fix from
commit ee2ab467 ("x86/boot: Use '-std=gnu11' to fix build with GCC 15")

Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Tested-by: Amit Machhiwal <amachhiw@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/20250331105722.19709-1-msuchanek@suse.de