[build] Disable gcc unterminated-string-initializer warnings

[build] Disable gcc unterminated-string-initializer warnings

GCC 15 generates a warning when a string initializer is too large to
allow for a trailing NUL terminator byte.  This type of initializer is
fairly common in signature strings such as ACPI table identifiers.

Fix by disabling the warning.

Signed-off-by: Michael Brown <mcb30@ipxe.org>