libpcap: fix build breakage with very high number of simultaneous jobs

libpcap: fix build breakage with very high number of simultaneous jobs

Building libpcap with high number (64) of simultaneous jobs fails:

 In file included from ./fmtutils.c:42:0:
 ./ftmacros.h:106:0: warning: "_BSD_SOURCE" redefined
   #define _BSD_SOURCE

 <command-line>:0:0: note: this is the location of the previous definition
 ./gencode.c:67:10: fatal error: grammar.h: No such file or directory
  #include "grammar.h"
           ^~~~~~~~~~~
 compilation terminated.
 Makefile:99: recipe for target 'gencode_pic.o' failed

So fix this by less intrusive way by disabling the parallel builds for
this package.

Ref: FS#3010
Signed-off-by: Petr Štetiar <ynezz@true.cz>