Put the project's own include directory ahead of the pkg-config flags.

libdndcp_la_CPPFLAGS listed @GTK_CPPFLAGS@ first, so every -I emitted by
"pkg-config --cflags gtk+-3.0" preceded the tree's own -I paths.  On a system
where that expands to -I/usr/include/json-c the plugin's

    #include "debug.h"

resolves to json-c's debug.h instead of open-vm-tools' lib/include/debug.h.
json-c's header does not declare Debug(), so every LOG() in the dndcp plugin
fails to compile:

    dndGuest/copyPasteRpcV4.cc:31:28: error: 'Debug' was not declared in this
    scope; did you mean 'g_debug'?

json-c reaches the gtk+-3.0 cflags through util-linux built with USE=cryptsetup:
mount.pc carries "Requires.private: blkid libcryptsetup" and libcryptsetup.pc
carries "Requires.private: ... json-c ...".  That is why the failure looks
compiler- or version-specific in bug reports while really depending on how
util-linux was built -- it reproduces identically on 13.0.10 and 13.1.0.

Only the search order changes; no flag is added or removed.

Bug: https://bugs.gentoo.org/956496
--- a/services/plugins/dndcp/Makefile.am
+++ b/services/plugins/dndcp/Makefile.am
@@ -23,6 +23,7 @@
 plugin_LTLIBRARIES = libdndcp.la
 
 libdndcp_la_CPPFLAGS =
+libdndcp_la_CPPFLAGS += -I$(top_srcdir)/lib/include
 libdndcp_la_CPPFLAGS += @GTK_CPPFLAGS@
 libdndcp_la_CPPFLAGS += @PLUGIN_CPPFLAGS@
 libdndcp_la_CPPFLAGS += -I$(top_srcdir)/services/plugins/dndcp/dnd