Ignore 'extern "C++"' section in a version script

This allows creating extern "C++" {} sections in version script in order
to export some of private platform symbols from NDK libraries.

Example for context: https://android.googlesource.com/platform/frameworks/native/+/3a11413a2584cf9a51e4315bef27546f602578fb/libs/nativewindow/include/private/android/AHardwareBufferHelpers.h

Bug: http://b/69603741
Test: make
Change-Id: I2f9e6678c0a6cd90113543e890786876d4c3e992