aarch64: Add cast for integer to enum cast

aarch64: Add cast for integer to enum cast

/home/simark/src/binutils-gdb/gdb/aarch64-linux-tdep.c: In function ‘int aarch64_linux_syscall_record(regcache*, long unsigned int)’:
/home/simark/src/binutils-gdb/gdb/aarch64-linux-tdep.c:904:57: error: invalid conversion from ‘long unsigned int’ to ‘aarch64_syscall’ [-fpermissive]
   syscall_gdb = aarch64_canonicalize_syscall (svc_number);
                                                         ^
/home/simark/src/binutils-gdb/gdb/aarch64-linux-tdep.c:645:1: note:   initializing argument 1 of ‘gdb_syscall aarch64_canonicalize_syscall(aarch64_syscall)’
 aarch64_canonicalize_syscall (enum aarch64_syscall syscall_number)
 ^

gdb/ChangeLog:

	* aarch64-linux-tdep.c (aarch64_linux_syscall_record): Add cast.