fuzz: add target for Descriptor::parse_descriptor

[brunoerg]

No description provided.

[apoelstra]

Looks like CI is failing because of clippy:

  --> fuzz/fuzz_targets/parse_descriptor_priv.rs:11:57
   |
11 |     if let Ok((desc, _)) = Descriptor::parse_descriptor(&secp, &data_str) {
   |                                                         ^^^^^ help: change this to: `secp`
   |

[brunoerg]

@apoelstra thanks, fixed!

[sanket1729]

ACK eb1e9f6

[apoelstra]

ACK eb1e9f6; successfully ran local tests; thanks!