package/libiconv: bump to 1.15

package/libiconv: bump to 1.15

libiconv doesn't build with gcc 6 due to a macro in stdio.h using gets()
which has been removed with the C11 standard.

_GL_WARN_ON_USE (gets, "gets is a security hole - use fgets instead");

With libiconv 1.15, this macro is protected by

 #if HAVE_RAW_DECL_GETS && !defined __cplusplus
 _GL_WARN_ON_USE (gets, "gets is a security hole - use fgets instead");
 #endif

Fixes:
http://autobuild.buildroot.net/results/61b/61bbfc839f99feb043d3456f107429d6e5319258

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>