Android: fixes build NDK 21d

[ericoporto]

Adds stdbool.h header to prevent build error with NDK 21d.

Description

Currently with NDK 21d I am hitting

  In file included from _deps/sdl2_content-src/src/audio/aaudio/SDL_aaudio.c:29:
  In file included from _deps/sdl2_content-src/src/audio/aaudio/SDL_aaudio.h:27:
  /opt/android-sdk-linux/ndk/21.3.6528147/toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/include/aaudio/AAudio.h:809:9: error: unknown type name 'bool'
          bool privacySensitive) __INTRODUCED_IN(30);
          ^
  /opt/android-sdk-linux/ndk/21.3.6528147/toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/include/aaudio/AAudio.h:1527:12: error: unknown type name 'bool'
  AAUDIO_API bool AAudioStream_isPrivacySensitive(AAudioStream* stream)

This appears to be a bug in NDK itself. It's currently possible to workaround by adding the header here in this PR.

Existing Issue(s)

android/ndk#1281