package/mg: needs a toolchain with wchar support

Since version 4.2, mg uses iswalnum() from <wctype.h> to tell whether
a UTF-8 character is part of a word (upstream commit ee22ef729b16
"Tell UTF-8 words by their Unicode class"). This breaks the build with
toolchains that lack wide character support, such as uClibc without
WCHAR:

utf8.c:23:10: fatal error: wctype.h: No such file or directory
   23 | #include <wctype.h>
      |          ^~~~~~~~~~

Add a dependency on BR2_USE_WCHAR, together with the matching comment.

This issue has been present since commit 3b5fbe8f6e ("package/mg:
bump to v4.4"), which updated mg from 4.1, so it pulled in this
change. That commit is not part of any release nor of any maintenance
branch yet, so only master is affected.

Fixes:
https://autobuild.buildroot.org/results/9d55c1b27627118c4279d9aade9f65b45db57e52

Fixes: 3b5fbe8f6e ("package/mg: bump to v4.4")
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/mg/Config.in

@@ -1,6 +1,7 @@
 config BR2_PACKAGE_MG
 	bool "mg"
 	depends on BR2_USE_MMU # fork()
+	depends on BR2_USE_WCHAR
 	help
 	  Emacs-like text editor (260 kiB)
 
@@ -16,3 +17,7 @@ config BR2_PACKAGE_MG
 	  and, in particular, no requirement on ncurses or termcap.
 
 	  https://github.com/troglobit/mg/
+
+comment "mg needs a toolchain w/ wchar"
+	depends on BR2_USE_MMU
+	depends on !BR2_USE_WCHAR