tcpdump/libpcap: correctly declare dependency between lex/yacc

* Previously, debugging jam, it would output a warning along the
  lines of 'warning: using independent target ... grammar.c'. To
  fix the dependency, it needs to be declared from object file
  to the generated source file.
* With the correct dependency (scanner.o on grammar.c), this
  warning is eliminated, and the dependency graph shown is now
  correct
* This should address #4091

src/bin/network/tcpdump/libpcap/Jamfile

@@ -27,5 +27,6 @@ StaticLibrary libpcap.a :
 	version.c
 ;
 
-# scanner.l includes the generated grammar.h
-Includes [ FGristFiles scanner.l ] : [ FGristFiles grammar.h ] ;
+# scanner.l includes the generated grammar.h, but we need to
+# declare the dependency from .o to .c
+Depends [ FGristFiles scanner.o ] : [ FGristFiles grammar.c ] ;