binutils/configure: look for msgpack-c.pc (in addition to msgpack.pc)

binutils/configure: look for msgpack-c.pc (in addition to msgpack.pc)

msgpack.pc was renamed to msgpack-c.pc in this commit [1].  This means
that we now find both in the wild.  For example Debian Bookworm has
msgpack.pc [2] while Debian Trixie has msgpack-c.pc [3].

Update the check in configure.ac to check for both.  Nothing in the code
needs to change.

[1] https://github.com/msgpack/msgpack-c/commit/01f3d24feee3a06b2a83c89b54fd0ec778d14610
[2] https://packages.debian.org/bookworm/amd64/libmsgpack-dev/filelist
[3] https://packages.debian.org/trixie/amd64/libmsgpack-c-dev/filelist