Fix find_path bug in CMakeLists

[jmazanec15]

Issue #, if available:
#279

Description of changes:
This PR fixes a build failure when using CMake >= 3.18. During the build process, we use find_path to check if the nmslib submodule has already been pulled. If it has not, we pull it (in line 37). Recently, Ubuntu 18 in Github actions upgraded from Cmake 3.17 to 3.19.

For some reason, I added the REQUIRED parameter in find_path. This had no effect in Cmake < 3.18. In Cmake >= 3.18, this causes CMake process to fail: https://cmake.org/cmake/help/v3.18/command/find_path.html. So, the fix is to remove REQUIRED parameter.

I verified the change works for CMake 3.19 on Ubuntu 18. I also verified that the fix does not break functionality for 3.17 on my Mac.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

Merging #280 (61fdd9d) into master (fd32d2f) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #280   +/-   ##
=========================================
  Coverage     79.34%   79.34%           
  Complexity      359      359           
=========================================
  Files            58       58           
  Lines          1404     1404           
  Branches        126      126           
=========================================
  Hits           1114     1114           
  Misses          242      242           
  Partials         48       48           

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[vamshin]

LGTM!