alsa-lib: fix build when dynamic load is not supported (e.g. blackfin FLAT)

The FLAT GNU toolchain doesn't include the dlfcn.h header file.
Provide the necessary declarations (RTLD_*) to make alsa-lib happy.

Fixes
http://autobuild.buildroot.org/results/706/7069e1f43cbed745d65f7dd9904a3fff034530ac/build-end.log



Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
[Thomas: change sequence number from 003 to 0003, update patch and commit
         message ]
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>