Description of problem:
$ pkgconf --cflags libassuan
(nothing)

This breaks dependent packages which try to DTRT and use pkgconf to
query compilation flags.

Version-Release number of selected component (if applicable):
libassuan-devel-2.5.3-1.fc31.x86_64

How reproducible:
deterministic

Expected results:
$ pkgconf --cflags libassuan
-I/usr/include/libassuan2

It seems /usr/lib64/pkgconfig/libassuan.pc already defines includedir, but does 
not use it for anything.
Suggested fix:
- Cflags: 
+ Cflags: -I${includedir}