linux-record.c:record_linux_system_call: fix -Wpointer-sign

linux-record.c:record_linux_system_call: fix -Wpointer-sign

$ make WERROR_CFLAGS="-Wpointer-sign -Werror" linux-record.o 2>&1 1>/dev/null
...
../../src/gdb/linux-record.c: In function ‘record_linux_system_call’:
../../src/gdb/linux-record.c:1152:9: error: pointer targets in passing argument 3 of ‘regcache_raw_read_signed’ differ in signedness [-Werror=pointer-sign]
In file included from ../../src/gdb/linux-record.c:23:0:
../../src/gdb/regcache.h:76:3: note: expected ‘long int *’ but argument is of type ‘long unsigned int *’
../../src/gdb/linux-record.c:1186:13: error: pointer targets in passing argument 3 of ‘regcache_raw_read_signed’ differ in signedness [-Werror=pointer-sign]
In file included from ../../src/gdb/linux-record.c:23:0:
../../src/gdb/regcache.h:76:3: note: expected ‘long int *’ but argument is of type ‘long unsigned int *’

I believe the read_signed calls are correct, and the variables are in
the wrong.

Tested on x86_64 Fedora 17.

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

	* linux-record.c (record_linux_system_call) <gdb_sys_msgrcv,
	RECORD_MSGRCV>: Pass a signed variable to
	regcache_raw_read_signed, instead of an unsigned one.