Add missing extern keyword for a couple of numutils functions

In passing, also remove a few surplus empty lines from pg_ltoa and
pg_ulltoa_n in numutils.c

Reported-by: Andrew Gierth
Discussion: https://postgr.es/m/87y2ou3xuh.fsf@news-spur.riddles.org.uk
Backpatch-through: 13, where these changes were introduced

src/backend/utils/adt/numutils.c

@@ -413,7 +413,6 @@ pg_ultoa_n(uint32 value, char *a)
 void
 pg_ltoa(int32 value, char *a)
 {
-
 	uint32		uvalue = (uint32) value;
 	int			len;
 
@@ -437,7 +436,6 @@ pg_ulltoa_n(uint64 value, char *a)
 				i = 0;
 	uint32		value2;
 
-
 	/* Degenerate case */
 	if (value == 0)
 	{

src/include/utils/builtins.h

@@ -48,8 +48,8 @@ extern int32 pg_atoi(const char *s, int size, int c);
 extern int16 pg_strtoint16(const char *s);
 extern int32 pg_strtoint32(const char *s);
 extern void pg_itoa(int16 i, char *a);
-int			pg_ultoa_n(uint32 l, char *a);
-int			pg_ulltoa_n(uint64 l, char *a);
+extern int pg_ultoa_n(uint32 l, char *a);
+extern int pg_ulltoa_n(uint64 l, char *a);
 extern void pg_ltoa(int32 l, char *a);
 extern void pg_lltoa(int64 ll, char *a);
 extern char *pg_ultostr_zeropad(char *str, uint32 value, int32 minwidth);