Add missing prototype for Tcldom_libxml2_Init.  Required
for building in strict C99 mode.

Submitted upstream: <https://sourceforge.net/p/tclxml/patches/29/>

diff --git a/include/tcldom-libxml2/tcldom-libxml2.h b/include/tcldom-libxml2/tcldom-libxml2.h
index 68202818b7d8e6c2..842df263520c7b16 100644
--- a/include/tcldom-libxml2/tcldom-libxml2.h
+++ b/include/tcldom-libxml2/tcldom-libxml2.h
@@ -66,6 +66,8 @@ extern "C" {
 # endif
 #endif
 
+EXTERN int Tcldom_libxml2_Init _ANSI_ARGS_((Tcl_Interp *));
+
 /*
  * The following function is required to be defined in all stubs aware
  * extensions of TclDOM.  The function is actually implemented in the stub