tools/libxl: Correct static pattern rule for pkgconfig files

tools/libxl: Correct static pattern rule for pkgconfig files

Attempting to build libxl causes Make to emit the following warnings

andrewcoop@andrewcoop:xen.git$ make -C tools/libxl all
...
Makefile:253: target `xenlight.pc' doesn't match the target pattern
Makefile:253: target `xlutil.pc' doesn't match the target pattern
...

because the static pattern rule is malformed.  'Makefile' as the only
prereq-pattern does not contain a pattern.

The rule ends up working because of the use of $@.in where $< should have been
used, but lacked any dependency between a $FOO.pc and its .in source file.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>