common: fix return value check for setpriority

[o7si]

On macOS and POSIX systems, a return value of 0 from getpriority() indicates success.

RETURN VALUES
     Since getpriority() can legitimately return the value -1, it is necessary to clear the external variable errno prior to
     the call, then check it afterward to determine if a -1 is an error or a legitimate value.  The setpriority() call returns
     0 if there is no error, or -1 if there is.

The ! operator in the code should be removed.

llama.cpp/common/common.cpp

Lines 254 to 257 in 06705fd

	if (!setpriority(PRIO_PROCESS, 0, p)) {
	LOG_WRN("failed to set process priority %d : %s (%d)\n", prio, strerror(errno), errno);
	return false;
	}

Additionally, the Windows version's logic for SetPriorityClass is correct, since it returns 0 to indicate failure.

llama.cpp/common/common.cpp

Lines 228 to 231 in 06705fd

	if (!SetPriorityClass(GetCurrentProcess(), p)) {
	LOG_WRN("failed to set process priority class %d : (%d)\n", prio, (int) GetLastError());
	return false;
	}