stap-probe.c: fix -Wpointer-sign

stap-probe.c: fix -Wpointer-sign

$ make WERROR_CFLAGS="-Wpointer-sign -Werror" stap-probe.o 2>&1 1>/dev/null
../../src/gdb/stap-probe.c: In function ‘handle_stap_probe’:
../../src/gdb/stap-probe.c:1306:19: error: pointer targets in assignment differ in signedness [-Werror=pointer-sign]
cc1: all warnings being treated as errors
make: *** [stap-probe.o] Error 1

provider is a string, so it's rightfully a char*.  'data' holds raw
bytes (bfd_byte), so a cast is the right thing to do.

gdb/
2013-03-07  Pedro Alves  <palves@redhat.com>

	* stap-probe.c (handle_stap_probe): Add cast to char*.