glib-2.0: Specify libiconv when building on uclibc

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
|   ^

(From OE-Core rev: 38b6c4df7c215ed7fd6be107fbc2527e66791e2e)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>