doc/rrdbuild: Add 'dc' to Debian build dependencies

[diederikdehaas]

Without dc the create-with-source-4 test fails.

[codecov]

Codecov Report

Merging #1224 (aea2b9b) into master (97e0c81) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1224      +/-   ##
==========================================
+ Coverage   50.92%   50.95%   +0.03%     
==========================================
  Files          45       45              
  Lines       18092    18093       +1     
==========================================
+ Hits         9213     9219       +6     
+ Misses       8879     8874       -5     

see 2 files with indirect coverage changes

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[diederikdehaas]

It turns out this is the last dependency needed. After installing dc I could build all the Debian rrd-tool packages, so not only make check, but the full build succeeded.