Fix compilation of gdb/arm-linux-nat.c on Raspbian GNU/Linux 7

Fix compilation of gdb/arm-linux-nat.c on Raspbian GNU/Linux 7

GDB trunk fails to compile on Raspbian GNU/Linux 7 because
PTRACE_GETREGSET and PTRACE_SETREGSET are not defined in sys/ptrace.h.

gcc -g -O2   -I. -I. -I./common -I./config -DLOCALEDIR="\"/usr/local/share/locale\"" -DHAVE_CONFIG_H -I./../include/opcode -I./../opcodes/.. -I./../readline/.. -I./../zlib -I../bfd -I./../bfd -I./../include -I../libdecnumber -I./../libdecnumber  -I./gnulib/import -Ibuild-gnulib/import   -DTUI=1  -Wall -Wpointer-arith -Wno-unused -Wunused-value -Wunused-function -Wno-switch -Wno-char-subscripts -Wempty-body -Wpointer-sign -Wmissing-prototypes -Wdeclaration-after-statement -Wmissing-parameter-type -Wold-style-declaration -Wold-style-definition -Wformat-nonliteral -Werror -c -o arm-linux-nat.o -MT arm-linux-nat.o -MMD -MP -MF .deps/arm-linux-nat.Tpo arm-linux-nat.c
arm-linux-nat.c: In function 'fetch_fpregister':
arm-linux-nat.c:103:21: error: 'PTRACE_GETREGSET' undeclared (first use in this function)
arm-linux-nat.c:103:21: note: each undeclared identifier is reported only once for each function it appears in
arm-linux-nat.c: In function 'fetch_fpregs':
arm-linux-nat.c:144:21: error: 'PTRACE_GETREGSET' undeclared (first use in this function)
arm-linux-nat.c: In function 'store_fpregister':
arm-linux-nat.c:184:21: error: 'PTRACE_GETREGSET' undeclared (first use in this function)
arm-linux-nat.c:211:21: error: 'PTRACE_SETREGSET' undeclared (first use in this function)
...

This patch includes the gdb header file nat/linux-ptrace.h, which provides
fallback definitions.

2015-06-16  Martin Simmons  <martin@lispworks.com>  (tiny patch)

	* arm-linux-nat.c: Include nat/linux-ptrace.h.