change FlexLexer.h

src/meson.build

@@ -20,7 +20,7 @@ if not cpp_compiler.has_header('FlexLexer.h')
     	  fs.parent(fs.parent(flex_exe.full_path())) / 'include' / 'FlexLexer.h', 
     	  'FlexLexer.h'
 	  )
-  flexlexer_h = 'FlexLexer.h'
+  flexlexer_h = '../src/FlexLexer.h'
 else
   flexlexer_h = []
 endif