lvm2: disable build with uClibc-ng

lvm2: disable build with uClibc-ng

lvm2 needs floorl() amnd nearbyintl(), which uClibc does not provide
for all architectures.

The simplest solution is to just disable lvm2 for uClibc.

Fixes:
  http://autobuild.buildroot.net/results/733/733c4d52f22d5f104b835d0f1c64e8a555ec1339/
  http://autobuild.buildroot.net/results/9ce/9ce371b59f0113d10261ede919a7e4f2ce377b3c/
  http://autobuild.buildroot.net/results/e48/e48fbec55160428b84c8d3b2ca957e8f6432c8e1/

Cc: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Thomas: adjust commit log, as suggested by Yann E. Morin.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>