When selecting the SHA1 backend, we only include the respective C
implementation of the selected backend. But since commit bd48bf3f
(hash: introduce source files to break include circles, 2019-06-14), we
have introduced separate headers and compilation units for all hashes.
So by not including the headers, we may not honor them to compute
whether a file needs to be recompiled and they also will not be
displayed in IDEs.

Add the header files to fix this problem.