h5ltget_attribute_f does not actually support retrieval of CHARACTERs

**Describe the bug**
The HL Fortran routine `h5ltget_attribute_f()` advertises that it can retrieve INTEGERs, REALs, and CHARACTERs but attempting to retrieve it for a string (CHARACTERS) does not actually modify the supplied buffer. The Fortran version of the routine does look at the first character of the `buf_type` argument and checks for 'C'. But the C version of the routine, `h5ltget_attribute_c()` in hl/fortran/src/H5LTfc.c, does not have a corresponding code block for 'C' and simply returns without doing anything.

**Expected behavior**
The HL Fortran routine `h5ltget_attribute_f()` can retrieve a string (CHARCTER).

**Platform (please complete the following information)**
- Found in HDF5 version 1.14.6 but I've confirmed the issue is still present in 'develop'.