xtensa-xtregs: Constify field

xtensa-xtregs: Constify field

Fix:

In file included from /home/emaisin/src/binutils-gdb/gdb/xtensa-linux-nat.c:46:0:
/home/emaisin/src/binutils-gdb/gdb/xtensa-xtregs.c:37:1: error: ISO C++ forbids converting a string constant to 'char*' [-Werror=write-strings]
 };
 ^

gdb/ChangeLog:

	* xtensa-xtregs.c (xtensa_regtable_t) <name>: Constify.