Suppress warnings from `find_package(fmt)` (#1822)
Suppress warnings from `find_package(fmt)` (#1822)

It's not an error if we are unable to find the `fmt` package when building
the Spdlog external project. This changes hides:
```
CMake Warning at cmake/Modules/FindSpdlog_EP.cmake:70 (find_package):
  By not providing "Findfmt.cmake" in CMAKE_MODULE_PATH this project has
  asked CMake to find a package configuration file provided by "fmt", but
  CMake did not find one.
  Could not find a package configuration file provided by "fmt" with any of
  the following names:
    fmtConfig.cmake
    fmt-config.cmake
  Add the installation prefix of "fmt" to CMAKE_PREFIX_PATH or set "fmt_DIR"
  to a directory containing one of the above files.  If "fmt" provides a
  separate development package or SDK, be sure it has been installed.
Call Stack (most recent call first):
  tiledb/CMakeLists.txt:393 (find_package)
```

Co-authored-by: Joe Maley <16a9a54ddf4259952e3c118c763138e83693d7fd@tiledb.com>
TileDB-Inc/TileDB,TileDB-Inc/TileDB,TileDB-Inc/TileDB,TileDB-Inc/TileDB