Fix cast to 'gdb_breakpoint *'

Fix cast to 'gdb_breakpoint *'

c-mode buildbot catches a build problem,

gcc -g -O2    -I. -I../../../binutils-gdb/gdb/gdbserver -I../../../binutils-gdb/gdb/gdbserver/../common -I../../../binutils-gdb/gdb/gdbserver/../regformats -I../../../binutils-gdb/gdb/gdbserver/.. -I../../../binutils-gdb/gdb/gdbserver/../../include -I../../../binutils-gdb/gdb/gdbserver/../gnulib/import -Ibuild-gnulib-gdbserver/import  -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 -Wno-missing-prototypes -Werror -DGDBSERVER -c -o hostio.o -MT hostio.o -MMD -MP -MF .deps/hostio.Tpo ../../../binutils-gdb/gdb/gdbserver/hostio.c
../../../binutils-gdb/gdb/gdbserver/mem-break.c: In function find_gdb_breakpoint:
../../../binutils-gdb/gdb/gdbserver/mem-break.c:996:15: error: gdb_breakpoint undeclared (first use in this function)
       return (gdb_breakpoint *) bp;

we should use 'struct gdb_breakpoint' rather than 'gdb_breakpoint'.
Patch below fixes this.

gdb/gdbserver:

2016-07-21  Yao Qi  <yao.qi@linaro.org>

	* mem-break.c (find_gdb_breakpoint): Cast bp to
	'struct gdb_breakpoint *' rather than 'gdb_breakpoint *'.