libunistring: add wchar dependency

Even though libunistring itself builds fine, its library contains
undefined references to wchar related functions, which are causing
link failures when the library is actually linked into an application.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/libunistring/Config.in

@@ -1,8 +1,12 @@
 config BR2_PACKAGE_LIBUNISTRING
 	bool "libunistring"
+	depends on BR2_USE_WCHAR
 	help
 	  The libunistring library provides functions for manipulating
 	  Unicode strings and for manipulating C strings according to
 	  the Unicode standard.
 
 	  http://www.gnu.org/software/libunistring/
+
+comment "libunistring needs a toolchain w/ wchar"
+	depends on !BR2_USE_WCHAR