fix compile error

[zhjwpku]

I got the following compile error, this patch add libaio to the target's link libraries.

[ 43%] Linking CXX executable build_memory_index
/usr/bin/ld: ../src/libdiskann.so: undefined reference to `io_submit'
/usr/bin/ld: ../src/libdiskann.so: undefined reference to `io_getevents'
/usr/bin/ld: ../src/libdiskann.so: undefined reference to `io_setup'
/usr/bin/ld: ../src/libdiskann.so: undefined reference to `io_destroy'
collect2: error: ld returned 1 exit status
make[2]: *** [apps/CMakeFiles/build_memory_index.dir/build.make:99: apps/build_memory_index] Error 1
make[1]: *** [CMakeFiles/Makefile2:304: apps/CMakeFiles/build_memory_index.dir/all] Error 2
make: *** [Makefile:136: all] Error 2

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• [ ] Does this PR add any new dependencies?
• [ ] Does this PR modify any existing APIs?
  • [ ] Is the change to the API backwards compatible?
• [ ] Should this result in any changes to our documentation, either updating existing docs or adding new ones?

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