move and deduplicate declarations of __dns_parse to make it checkable

the source file for this function is completely standalone, but it
doesn't seem worth adding a header just for it, so declare it in
lookup.h for now.

src/network/dns_parse.c

 #include <string.h>
+#include "lookup.h"
 
 int __dns_parse(const unsigned char *r, int rlen, int (*callback)(void *, int, const void *, int, const void *), void *ctx)
 {

src/network/getnameinfo.c

@@ -10,7 +10,6 @@
 #include "lookup.h"
 #include "stdio_impl.h"
 
-int __dns_parse(const unsigned char *, int, int (*)(void *, int, const void *, int, const void *), void *);
 int __dn_expand(const unsigned char *, const unsigned char *, const unsigned char *, char *, int);
 int __res_mkquery(int, const char *, int, int, const unsigned char *, int, const unsigned char*, unsigned char *, int);
 int __res_send(const unsigned char *, int, unsigned char *, int);

src/network/lookup.h

@@ -36,4 +36,6 @@ int __lookup_ipliteral(struct address buf[static 1], const char *name, int famil
 
 int __get_resolv_conf(struct resolvconf *, char *, size_t);
 
+int __dns_parse(const unsigned char *, int, int (*)(void *, int, const void *, int, const void *), void *);
+
 #endif

src/network/lookup_name.c

@@ -98,7 +98,6 @@ struct dpc_ctx {
 	int cnt;
 };
 
-int __dns_parse(const unsigned char *, int, int (*)(void *, int, const void *, int, const void *), void *);
 int __dn_expand(const unsigned char *, const unsigned char *, const unsigned char *, char *, int);
 int __res_mkquery(int, const char *, int, int, const unsigned char *, int, const unsigned char*, unsigned char *, int);
 int __res_msend_rc(int, const unsigned char *const *, const int *, unsigned char *const *, int *, int, const struct resolvconf *);