cmake: lemon: fix path to internal lemon tool

Wireshark fails to build when cross-compiling on Gentoo/ChromeOS
systems because the lemon command is not properly specified or
included in PATH, failing with:

/bin/sh: lemon: command not found

The relevant excerpt from build.ninja is:

COMMAND = cd ..._build/plugins/epan/mate && lemon -T.../tools/lemon/lempar.c
-d. .../plugins/epan/mate/mate_grammar.lemon

By specifying the full path to "lemon" we ensure it is
always searched and found in the correct location.

cmake/modules/UseLemon.cmake

@@ -13,7 +13,7 @@ MACRO(ADD_LEMON_FILES _source _generated)
           # These files are generated as side-effect
           ${_out}.h
           ${_out}.out
-         COMMAND lemon
+         COMMAND $<TARGET_FILE:lemon>
            -T${_lemonpardir}/lempar.c
            -d.
            ${_in}