Source: Fix -Wmissing-variable-declarations warning

Source/cm_codecvt.cxx

diff --git a/Source/cm_codecvt.cxx b/Source/cm_codecvt.cxx
index ddee968..8115306 100644
--- a/Source/cm_codecvt.cxx
+++ b/Source/cm_codecvt.cxx
@@ -9,11 +9,8 @@
 #  include <windows.h>
 #  undef max
 #  include "cmsys/Encoding.hxx"
-#endif
 
-#if defined(_WIN32)
-/* Number of leading ones before a zero in the byte (see cm_utf8.c).  */
-extern "C" unsigned char const cm_utf8_ones[256];
+#  include "cm_utf8.h"
 #endif
 
 codecvt::codecvt(Encoding e)

Source/cm_utf8.h

diff --git a/Source/cm_utf8.h b/Source/cm_utf8.h
index fa9ed3a..67f3d3f 100644
--- a/Source/cm_utf8.h
+++ b/Source/cm_utf8.h
@@ -6,6 +6,8 @@
 extern "C" {
 #endif
 
+extern unsigned char const cm_utf8_ones[256];
+
 /** Decode one UTF-8 character from the input byte range.  On success,
     stores the unicode character number in *pc and returns the first
     position not extracted.  On failure, returns 0.  */