HBSD: fix build warnings, which caused by -Wunused-function

--8<--
/usr/src/sys/sys/pax.h:119:13: warning: unused function 'pax_aslr_init_prison' [-Wunused-function]
static void pax_aslr_init_prison(struct prison *pr __unused) {}
            ^
/usr/src/sys/sys/pax.h:120:13: warning: unused function 'pax_aslr_init_prison32' [-Wunused-function]
static void pax_aslr_init_prison32(struct prison *pr __unused) {}
            ^
/usr/src/sys/sys/pax.h:146:13: warning: unused function 'pax_segvguard_init_prison' [-Wunused-function]
static void pax_segvguard_init_prison(struct prison *pr __unused) {}
            ^
/usr/src/sys/sys/pax.h:161:13: warning: unused function 'pax_hardening_init_prison' [-Wunused-function]
static void pax_hardening_init_prison(struct prison *pr __unused) {}
            ^
/usr/src/sys/sys/pax.h:173:13: warning: unused function 'pax_ptrace_hardening_init_prison' [-Wunused-function]
static void pax_ptrace_hardening_init_prison(struct prison *pr __unused) {}
--8<--

github-issue: #60 #76
Signed-off-by: Oliver Pinter <oliver.pinter@hardenedbsd.org>