[meson] Declare emper_include in root meson.build file

It appears that otherwise the "emper_include += directory(.)" in
include/meson.build has no effect.

emper/meson.build

@@ -29,6 +29,7 @@ emper_cpp_sources = [
 
 emper_generated_files = []
 
+emper_include = []
 subdir('include')
 
 emper_library_include = []