uclibc: explicitly set HAVE_SHARED depending on BR2_PREFER_STATIC_LIB

Now with change of BR2_PREFER_STATIC_LIB meaning to "do not build dynamic libs
and build statically linked applications" it's possible to disable support of
shared libs in uClibc as well as builting of shared libc libs.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>

Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Cc: Peter Korsgaard <peter@korsgaard.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>