Fix gdb error: Undefined command: "Printf".

The swig.gdb file provides the swigprint and locswigprint user-defined
commands which stopped working somewhere around gdb 8.0 - 8.2.
```
(gdb) swigprint n
Undefined command: "Printf".  Try "help".
(gdb) locswigprint n
Undefined command: "Printf".  Try "help".
(gdb)
```

I couldn't find any mention of gdb's Printf, but replacing Printf with
printf works and is documented at
https://sourceware.org/gdb/download/onlinedocs/gdb/Output.html#Output

Fixes issue #1420

[skip ci]

CHANGES.current

@@ -7,6 +7,13 @@ the issue number to the end of the URL: https://github.com/swig/swig/issues/
 Version 4.0.0 (in progress)
 ===========================
 
+2019-01-18: wsfulton
+            #1420 Fix gdb debugger functions 'swigprint' and 'locswigprint' from swig.gdb to
+            work with newer versions of gdb-8. Fixes errors when debugging SWIG source with gdb:
+
+              (gdb) swigprint n
+              Undefined command: "Printf".  Try "help".
+
 2019-01-16: wsfulton
             Python static method wrapper changes
 
@@ -36,12 +43,10 @@ Version 4.0.0 (in progress)
                   ...
                   statmethod = staticmethod(_example.Klass_statmethod)
 
-
               - Modernise wrappers for static methods to use decorator syntax - @staticmethod.
 
               - Add missing runtime test for static class methods and using the actual class method.
 
-
 2019-01-12: ZackerySpytz
             [OCaml] #1403 #1194 Fix compilation problems for OCaml >= 4.03.0 due to OCaml using
             int64_t instead of int64.

Tools/swig.gdb

@@ -15,7 +15,7 @@ define swigprint
     else
         set $expand_count = -1
     end
-    Printf "%s\n", Swig_to_string($arg0, $expand_count)
+    printf "%s\n", Swig_to_string($arg0, $expand_count)
 end
 document swigprint
 Displays any SWIG DOH object
@@ -31,7 +31,7 @@ define locswigprint
     else
         set $expand_count = -1
     end
-    Printf "%s\n", Swig_to_string_with_location($arg0, $expand_count)
+    printf "%s\n", Swig_to_string_with_location($arg0, $expand_count)
 end
 document locswigprint
 Displays any SWIG DOH object prefixed with file and line location