Fix macOS build.

  system/core/adb/client/usb_libusb.cpp:162:20: error: unused function 'get_device_serial_path' [-Werror,-Wunused-function]
  static std::string get_device_serial_path(libusb_device* device) {
                     ^

Bug: N/A
Test: no choice but to submit and see...
Change-Id: I0026d3273891588c27df5ccd71c41ad9907b038d
diff --git a/client/usb_libusb.cpp b/client/usb_libusb.cpp
index a32c605..7539e5a 100644
--- a/client/usb_libusb.cpp
+++ b/client/usb_libusb.cpp
@@ -159,6 +159,7 @@
                         libusb_get_device_address(device));
 }
 
+#if defined(__linux__)
 static std::string get_device_serial_path(libusb_device* device) {
     uint8_t ports[7];
     int port_count = libusb_get_port_numbers(device, ports, 7);
@@ -172,6 +173,7 @@
     path += "/serial";
     return path;
 }
+#endif
 
 static bool endpoint_is_output(uint8_t endpoint) {
     return (endpoint & LIBUSB_ENDPOINT_DIR_MASK) == LIBUSB_ENDPOINT_OUT;