fix ifdefs for non-windows systems without long double

[chuckadams]

In ext/c/ffi.c, there is an #ifdef on line 70:

#ifdef HAVE_LONG_DOUBLE
	ZEND_FFI_TYPE_LONGDOUBLE,
#endif

Then later on line 315:

#ifndef PHP_WIN32
	case ZEND_FFI_TYPE_LONGDOUBLE:
		t->elements[i] = &ffi_type_longdouble;
		break;
#endif

A similar inconsistency occurs on line 362. This should be changed to #ifdef HAVE_LONG_DOUBLE, else it will fail to compile on any non-windows system when HAVE_LONG_DOUBLE is undefined.

This doesn't actually come up in normal compilation on Linux, only when I was fiddling with the build system and neglected to include the proper headers. So in some sense it's developer error, but it still ought to be corrected.