uci: macro uci_alloc_element not in uci.h

uci: macro uci_alloc_element not in uci.h

The macro uci_alloc_element is in the public header file uci.h. However,
the macros output refers to uci_alloc_generic wich is in uci_internal.h
and not public. Thus, uci_alloc_element should be private as well and
moved to uci_internal.h.

Signed-off-by: Jan Venekamp <jan@venekamp.net>