When building with clang, disable -Wformat-security for

When building with clang, disable -Wformat-security for
sys/dev/hpt27xx/osm_bsd.c, since it gets the following warnings:

sys/dev/hpt27xx/osm_bsd.c:1180:25: error: format string is not a string literal (potentially insecure) [-Werror,-Wformat-security]
            S_IRUSR | S_IWUSR, driver_name);
                               ^~~~~~~~~~~
@/dev/hpt27xx/hpt27xx_config.h:46:21: note: expanded from:
#define driver_name hpt27xx_driver_name
                    ^~~~~~~~~~~~~~~~~~~

Since 'hpt27xx_driver_name' is a constant string symbol (coming from the
proprietary hpt27xx_lib.o file), there is no security problem.

Because this driver is provided by the vendor, and applying changes
requires re-certification and other bureaucratic exercises, just disable
the warning for now.

MFC after:	1 week