parisc: fix building with gcc-15

The decompressor is built with the default C dialect, which is now gnu23
on gcc-15, and this clashes with the kernel's bool type definition:

In file included from include/uapi/linux/posix_types.h:5,
                 from arch/parisc/boot/compressed/misc.c:7:
include/linux/stddef.h:11:9: error: cannot use keyword 'false' as enumeration constant
   11 |         false   = 0,

Add the -std=gnu11 argument here, as we do for all other architectures.

Cc: stable@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Helge Deller <deller@gmx.de>