Fix typo in libsqlite flags variable

[polyrand]

The build.rs script searches for LIBSQLITE3_FLAGS, but the README mentions LIBSQLITE_FLAGS (missing 3).

[codecov]

Codecov Report

Merging #1115 (7132e4d) into master (ddb7141) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1115   +/-   ##
=======================================
  Coverage   78.34%   78.34%           
=======================================
  Files          47       47           
  Lines        5808     5808           
=======================================
  Hits         4550     4550           
  Misses       1258     1258           

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[gwenn]

Thanks and sorry for the long delay.