package/at: fix glibcism

at needs __isleap() which is in (e)glibc and uClibc, but not in musl.

Add a patch (from Open Embedded) that removes use of the macros, and
directly expands the corresponding code in lieu of the macro calls.

Fixes:
    http://autobuild.buildroot.org/results/6f2/6f25e80e8aedec91323ef9b67576b550d7abee60/
    http://autobuild.buildroot.org/results/da5/da5bb779f3077b079203fcbdb2533dd790963cd4/
    http://autobuild.buildroot.org/results/8a6/8a6ce608c6f46d08ed77049f796e98e6e10a1709/


    ...

Thanks to Jörg for pointing to the OE patch! :-)

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Jörg Krause <joerg.krause@embedded.rocks>
Tested-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>