asterisk-15.x: fix iconv related build issue

asterisk-15.x: fix iconv related build issue

libmariadb 10.2 needs to be linked in together with iconv. Include nls.mk
to help the build system find the iconv library.

Also update func_iconv's depend. We want to use the variable
$(ICONV_DEPENDS) provided by nls.mk. It resolves to
+BUILD_NLS:libiconv-full. This cannot be handled properly right now, as the
module depends filter cannot handle +<SYMBOL>:<package> depends. So add
another field to the Makefile's module function for complex depends, which
are passed on unfiltered.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>