linux-mips-low.c: Add casts

linux-mips-low.c: Add casts

Fixes a bunch of:

/home/simark/src/binutils-gdb/gdb/gdbserver/linux-mips-low.c: In function ‘void mips_store_fpregset(regcache*, const void*)’:
/home/simark/src/binutils-gdb/gdb/gdbserver/linux-mips-low.c:809:39: error: invalid conversion from ‘const void*’ to ‘const mips_register*’ [-fpermissive]
   const union mips_register *regset = buf;
                                       ^

gdb/gdbserver/ChangeLog:

	* linux-mips-low.c (mips_fill_gregset): Add cast.
	(mips_store_gregset): Likewise.
	(mips_fill_fpregset): Likewise.
	(mips_store_fpregset): Likewise.