librtlsdr: disable shared library target

Disable shared library target if BUILD_SHARES_LIBS is OFF.

Fixes:
http://autobuild.buildroot.net/results/d68/d68a23e2a57fd5430d74b8151931f36959d9578d//



Signed-off-by: Yuvaraj Patil <yuvaraj.patil@wipro.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>