Fix pc file to allow compiling with mtdev installed in non-std path.
Fix pc file to allow compiling with mtdev installed in non-std path.

The pkgconfig file does not specify the -I flag needed to be able to
compile with `pkg-config mtdev --cflags` when libmtdev is installed to
a directory that is not searched by the C compiler by default.

This patch fixes the issue.

Signed-off-by: David Kozub <0389d5459ac44801a6dd8028dac3544685af01d2@linux.fjfi.cvut.cz>
Signed-off-by: Henrik Rydberg <b7088a9c70345a9ab94cacab3cc3888d0a7dc884@euromail.se>

jiixyj/mtdev,whot/mtdev,Distrotech/mtdev,whot/mtdev,jiixyj/mtdev,Distrotech/mtdev