Fix compile issue when using NDK Unified Headers

jni/Application.mk

@@ -1,3 +1,4 @@
 APP_ABI := x86 x86_64 armeabi-v7a arm64-v8a
 APP_PLATFORM := android-21
+APP_UNIFIED_HEADERS := true
 APP_CPPFLAGS += -std=c++11

jni/magisk.h

@@ -23,6 +23,10 @@
 
 #define LOG_TAG    "Magisk"
 
+#ifndef ARG_MAX
+#define ARG_MAX 4096
+#endif
+
 // Global handler for PLOGE
 extern __thread void (*err_handler)(void);