[libc] Fix allocator inclusion

Previously, allocator functions were only available if they were included
from scudo or by using the system libc headers (i.e. by turning off the
full build). This patch changes the logic to include the prototypes for
the allocator functitons in all cases, which allows the linker to link
in the system's allocator.

Reviewed By: sivachandra, abrachet

Differential Revision: https://reviews.llvm.org/D119587