Fix duckdb_adbc_init signature

[hannes]

CC @krlmlr this one was in the R package at least

[codecov-commenter]

Codecov Report

Merging #33 (9a23ee3) into main (eb1e9e8) will increase coverage by 0.12%.
Report is 48 commits behind head on main.
The diff coverage is 85.71%.

❗ Current head 9a23ee3 differs from pull request most recent head bbd54af. Consider uploading reports for the commit bbd54af to get more accurate results

@@            Coverage Diff             @@
##             main      #33      +/-   ##
==========================================
+ Coverage   85.62%   85.75%   +0.12%     
==========================================
  Files         106      106              
  Lines        3597     3601       +4     
==========================================
+ Hits         3080     3088       +8     
+ Misses        517      513       -4     

Files	Coverage Δ
R/backend-dbplyr__duckdb_connection.R	92.63% <ø> (ø)
R/dbIsValid__duckdb_driver.R	100.00% <ø> (ø)
.../src/include/duckdb/common/arrow/arrow_wrapper.hpp	100.00% <ø> (ø)
src/duckdb/src/include/duckdb/common/exception.hpp	69.23% <ø> (ø)
src/duckdb/src/include/duckdb/common/types.hpp	100.00% <ø> (ø)
.../src/include/duckdb/common/types/validity_mask.hpp	97.50% <100.00%> (+0.13%)	⬆️
.../duckdb/src/include/duckdb/common/types/vector.hpp	80.95% <ø> (ø)
...b/src/include/duckdb/function/replacement_scan.hpp	100.00% <ø> (ø)
R/relational.R	80.00% <75.00%> (-1.58%)	⬇️

... and 4 files with indirect coverage changes

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