jit/118780 - make sure to include dlfcn.h when plugin support is disabled

jit/118780 - make sure to include dlfcn.h when plugin support is disabled

The following makes the dlfcn.h explicitly requested which avoids
build failure when JIT is enabled but plugin support disabled as
currently the include is conditional on plugin support.

	PR jit/118780
gcc/
	* system.h: Check INCLUDE_DLFCN_H for including dlfcn.h instead
	of ENABLE_PLUGIN.
	* plugin.cc: Define INCLUDE_DLFCN_H.

gcc/jit/
	* jit-playback.cc: Define INCLUDE_DLFCN_H.
	* jit-result.cc: Likewise.

(cherry picked from commit e22962538f64bb6e5ac87977ec8a5d86f4ef21cb)