FindIconv: Fix finding iconv.h on Alpine Linux

Alpine Linux has the `iconv.h` belonging to the builtin `iconv` library
provided by musl C library in the standard directory (`/usr/include`).
However, when installing the GNU libiconv APK package (`gnu-libiconv`),
the GNU `libiconv` library has header located in
`/usr/include/gnu-libiconv` subdirectory.  Fix finding the header
matching the `libiconv` library.

Issue: #24695