rtpjitterbuffer: fix build error with gcc (Debian 4.9.2-21) 4.9.2

rtpjitterbuffer: fix build error with gcc (Debian 4.9.2-21) 4.9.2

Replace static constants with macros to make gcc happy

  CC       elements/elements_rtpjitterbuffer-rtpjitterbuffer.o
elements/rtpjitterbuffer.c:387:1: error: initializer element is not constant
 static const GstClockTime PCMU_BUF_DURATION = PCMU_BUF_MS * GST_MSECOND;
 ^
elements/rtpjitterbuffer.c:388:1: error: initializer element is not constant
 static const guint PCMU_BUF_SIZE = 64000 * PCMU_BUF_MS / 1000;
 ^
elements/rtpjitterbuffer.c:390:5: error: initializer element is not constant
     PCMU_BUF_CLOCK_RATE * PCMU_BUF_MS / 1000;