pc-bios/optionrom: compile with -Wno-array-bounds

pc-bios/optionrom: compile with -Wno-array-bounds

Avoids the following bogus warning:

pvh_main.c: In function ‘pvh_load_kernel’:
pvh_main.c:101:42: warning: array subscript 0 is outside array bounds of ‘uint16_t[0]’ {aka ‘short unsigned int[]’} [-Warray-bounds]
  101 |         uint32_t ebda_paddr = ((uint32_t)*((uint16_t *)EBDA_BASE_ADDR)) << 4;
      |                                          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Cc: qemu-stable@nongnu.org
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>