Fix compilation error of m_ipt.c with -Werror enabled

Commit (5a650703 Makefile: make warnings into
errors ) causes the following build error.

gcc -Wall -Wstrict-prototypes -Werror -Wmissing-prototypes
-Wmissing-declarations -Wold-style-definition -O2 -I../include
-DRESOLVE_HOSTNAMES -DLIBDIR=\"/usr/lib\" -DCONFDIR=\"/etc/iproute2\"
-D_GNU_SOURCE -DCONFIG_GACT -DCONFIG_GACT_PROB -DIPT_LIB_DIR=\"/lib/xtables\"
-DYY_NO_INPUT   -c -o m_ipt.o m_ipt.c
cc1: warnings being treated as errors
m_ipt.c:72: error: no previous prototype for 'xtables_register_target'
m_ipt.c:361: error: no previous prototype for 'build_st'
make[1]: *** [m_ipt.o] Error 1

This is fixed by adding the prototype in the header include/iptables.h

I am not sure if this is due to something wrong on my build system but I am
using current glibc 2.17.
Signed-off-by: Vijay Subramanian <subramanian.vijay@gmail.com>

include/iptables.h

@@ -143,6 +143,8 @@ extern int line;
 /* Your shared library should call one of these. */
 extern void register_match(struct iptables_match *me);
 extern void register_target(struct iptables_target *me);
+extern void xtables_register_target(struct iptables_target *me);
+extern int build_st(struct iptables_target *target, struct ipt_entry_target *t);
 
 extern struct in_addr *dotted_to_addr(const char *dotted);
 extern char *addr_to_dotted(const struct in_addr *addrp);