Add a variable to allow adding an extra include path for LSS.

I have a project where we are trying to include breakpad as a git
submodule. The only problem getting it to build with ndk-build is that
the linux_syscall_support.h header is missing. Normally, this header
would be downloaded by the depot_tools. But we cannot install
depot_tools on our CI machines.

So, we would like to include the header as a separate submodule in our
repo. The only change we would need to make to breakpad to make that
work is to add this optional variable to the C header search path. In
the normal case, this variable will not be set and nothing will be
different. But we can set the variable to allow us to build without
installing depot_tools.

I have manually tested this using a separate project that links to
breakpad in this way.

Change-Id: Id116b2af1c1ba0a08464c213d8982b24a1821ee7
Reviewed-on: https://chromium-review.googlesource.com/c/breakpad/breakpad/+/1930179
Reviewed-by: Mark Mentovai <mark@chromium.org>

android/google_breakpad/Android.mk

@@ -93,7 +93,8 @@ LOCAL_SRC_FILES := \
     src/common/linux/safe_readlink.cc
 
 LOCAL_C_INCLUDES        := $(LOCAL_PATH)/src/common/android/include \
-                           $(LOCAL_PATH)/src
+                           $(LOCAL_PATH)/src \
+                           $(LSS_PATH)
 
 LOCAL_EXPORT_C_INCLUDES := $(LOCAL_C_INCLUDES)
 LOCAL_EXPORT_LDLIBS     := -llog