botan: pass --prefix=/usr to fix .pc file

botan uses a non-autotools build system. We currently don't pass any
--prefix=/usr, and by default the prefix is set to /usr/local. This
doesn't cause a lot of visible issues because at install time, we pass
DESTDIR=$(TARGET_DIR)/usr (the build system doesn't make the
difference between DESTDIR and prefix).

However, the generated pkg-config .pc file is wrong, as it contains
prefix=/usr/local instead of prefix=/usr, which doesn't match where
the botan library and headers are installed. By passing --prefix=/usr,
we make sure that the .pc file is in line with where botan is
installed.

Fixes bug #7760

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>