gh-40911: correct .pc name for mtx
    
typo fix in the name


### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [ ] The title is concise and informative.
- [ ] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - #12345: short description why this is a dependency -->
<!-- - #34567: ... -->
    
URL: #40911
Reported by: Dima Pasechnik
Reviewer(s): Tobias Diez

Author: Release Manager

src/meson.build

@@ -152,7 +152,7 @@ if not m4rie.found()
   # For some reason, m4rie is not found via pkg-config on some systems (eg Conda)
   m4rie = cc.find_library('m4rie', required: not is_windows, disabler: true)
 endif
-mtx = dependency('mtx', required: false)
+mtx = dependency('libmtx', required: false)
 if not mtx.found()
   # fallback for older versions without pkg-config
   mtx = cc.find_library(