glib-2.0: Specify libiconv when building on uclibc
We use GNU libiconv seprate package on uclibc
specifying this makes the configure clear of not
trying to detect glibc/iconv and then also finding
libiconv in sysroot and confusing itself with errors like
| gconvert.c:66:2: error: #error GNU libiconv not in use but included iconv.h is from libiconv
| #error GNU libiconv not in use but included iconv.h is from libiconv
| ^
