Fix DragonFly build

[petk]

On DragonFly by default the BSD make is used with the CSH shell and the first prerequisite variable $< in Makefile doesn't work there. So, we can simplify this by simply repeating the filename here.

--- ext/opcache/jit/ir/gen_ir_fold_hash ---
cc -DIR_TARGET_X64 -DIR_PHP -DIR_PHP_MM=0 -o ext/opcache/jit/ir/gen_ir_fold_hash 
cc: fatal error: no input files
compilation terminated.
*** [ext/opcache/jit/ir/gen_ir_fold_hash] Error code 1

OS: DragonFly 6.4 and everything installed by default.

P.S.: The $< variable might work with patterns or in some other cases. And tests all pass ok so far.