Change pkg-config files so that both <et/com_err.h> and <com_err.h> work

Change pkg-config files so that both <et/com_err.h> and <com_err.h> work

Change the include path in the Cflags field so that #include
<lib/foo.h> and <foo.h> will work.  We had originally used a C flags
which allowed <foo.h> to work, but many applications (especially those
not using pkg-config) had been using the <lob/foo.h> formulation which
didn't require an explicit -I{$includedir} option to the C compiler.

If those applications then converted over to pkg-config, and the
e2fsprogs libraries were installed with a prefix other than /usr, so
that the header files were in some directory such as
/usr/local/include, a program that used #include <lib/foo.h> would
fail to compile.

So change the pkg-config files to include both -I{$includedir} and
-I{$includir}/lib.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>