Use local adb in portable builds

For non-Windows portable builds, use the absolute path to the adb
executable located in the same directory as scrcpy.

On Windows, just use "adb", which is sufficient to use the local one.

app/src/adb/adb.c

@@ -34,11 +34,22 @@ sc_adb_init(void) {
         return true;
     }
 
+#if !defined(PORTABLE) || defined(_WIN32)
     adb_executable = strdup("adb");
     if (!adb_executable) {
         LOG_OOM();
         return false;
     }
+#else
+    // For portable builds, use the absolute path to the adb executable
+    // in the same directory as scrcpy (except on Windows, where "adb"
+    // is sufficient)
+    adb_executable = sc_file_get_local_path("adb");
+    if (!adb_executable) {
+        // Error already logged
+        return false;
+    }
+#endif
 
     return true;
 }