build: build libbupsplit separately

build: build libbupsplit separately

It avoids gobject-introspection warnings:

src/libostree/bupsplit.h:42: Warning: OSTree: symbol='bupsplit_find_ofs': Unknown namespace for symbol 'bupsplit_find_ofs'
src/libostree/bupsplit.h:43: Warning: OSTree: symbol='bupsplit_selftest': Unknown namespace for symbol 'bupsplit_selftest'
src/libostree/bupsplit.h:33: Warning: OSTree: symbol='BUP_BLOBBITS': Unknown namespace for symbol 'BUP_BLOBBITS'
src/libostree/bupsplit.h:34: Warning: OSTree: symbol='BUP_BLOBSIZE': Unknown namespace for symbol 'BUP_BLOBSIZE'
src/libostree/bupsplit.h:35: Warning: OSTree: symbol='BUP_WINDOWBITS': Unknown namespace for symbol 'BUP_WINDOWBITS'
src/libostree/bupsplit.h:36: Warning: OSTree: symbol='BUP_WINDOWSIZE': Unknown namespace for symbol 'BUP_WINDOWSIZE'

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>