scons : add HAVE_DLOPEN to build environment

fixes dlopen issue caused by 57c57df7b4

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=54140

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Signed-off-by: José Fonseca <jfonseca@vmware.com>

src/mesa/SConscript

@@ -31,6 +31,7 @@ if env['platform'] == 'windows':
 else:
     env.Append(CPPDEFINES = [
         'IN_DRI_DRIVER', # enable the remap table (for DRI drivers)
+        ('HAVE_DLOPEN', '1'),
     ])
 
 #