rabbitmq-c: don't pass BUILD_SHARED_LIBS

The BUILD_SHARED_LIBS option is already handled by the cmake-package
infrastructure, so there is no need to pass it at the package level.

Signed-off-by: Jan Heylen <jan.heylen@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>