arm-linux-nat.c: Add cast

arm-linux-nat.c: Add cast

Fixes:

/home/simark/src/binutils-gdb/gdb/arm-linux-nat.c: In function ‘const target_desc* arm_linux_read_description(target_ops*)’:
/home/simark/src/binutils-gdb/gdb/../include/libiberty.h:711:38: error: invalid conversion from ‘void*’ to ‘char*’ [-fpermissive]
 # define alloca(x) __builtin_alloca(x)
                                      ^
/home/simark/src/binutils-gdb/gdb/arm-linux-nat.c:578:13: note: in expansion of macro ‘alloca’
       buf = alloca (VFP_REGS_SIZE);
             ^

gdb/ChangeLog:

	* arm-linux-nat.c (arm_linux_read_description): Add cast.