Fix symbol references to be cross-platform

For some reason, both ways work on Linux, but only this way works on
Windows, macOS, and FreeBSD.

src/StringEncodings.jl

@@ -62,13 +62,13 @@ show(io::IO, exc::T) where {T<:Union{IncompleteSequenceError,OutputBufferError}}
 
 function iconv_close(cd::Ptr{Nothing})
     if cd != C_NULL
-        ccall((:iconv_close, libiconv), Cint, (Ptr{Nothing},), cd) == 0 ||
+        ccall((:libiconv_close, libiconv), Cint, (Ptr{Nothing},), cd) == 0 ||
             throw(IConvError("iconv_close"))
     end
 end
 
 function iconv_open(tocode::String, fromcode::String)
-    p = ccall((:iconv_open, libiconv), Ptr{Nothing}, (Cstring, Cstring), tocode, fromcode)
+    p = ccall((:libiconv_open, libiconv), Ptr{Nothing}, (Cstring, Cstring), tocode, fromcode)
     if p != Ptr{Nothing}(-1)
         return p
     elseif errno() == EINVAL
@@ -131,7 +131,7 @@ function iconv!(cd::Ptr{Nothing}, inbuf::Vector{UInt8}, outbuf::Vector{UInt8},
     inbytesleft_orig = inbytesleft[]
     outbytesleft[] = BUFSIZE
 
-    ret = ccall((:iconv, libiconv), Csize_t,
+    ret = ccall((:libiconv, libiconv), Csize_t,
                 (Ptr{Nothing}, Ptr{Ptr{UInt8}}, Ref{Csize_t}, Ptr{Ptr{UInt8}}, Ref{Csize_t}),
                 cd, inbufptr, inbytesleft, outbufptr, outbytesleft)
 
@@ -163,7 +163,7 @@ function iconv_reset!(s::Union{StringEncoder, StringDecoder})
 
     s.outbufptr[] = pointer(s.outbuf)
     s.outbytesleft[] = BUFSIZE
-    ret = ccall((:iconv, libiconv), Csize_t,
+    ret = ccall((:libiconv, libiconv), Csize_t,
                 (Ptr{Nothing}, Ptr{Ptr{UInt8}}, Ref{Csize_t}, Ptr{Ptr{UInt8}}, Ref{Csize_t}),
                 s.cd, C_NULL, C_NULL, s.outbufptr, s.outbytesleft)
 
@@ -529,7 +529,7 @@ encode(s::AbstractString, enc::AbstractString) = encode(s, Encoding(enc))
 
 function test_encoding(enc::String)
     # We assume that an encoding is supported if it's possible to convert from it to UTF-8:
-    cd = ccall((:iconv_open, libiconv), Ptr{Nothing}, (Cstring, Cstring), enc, "UTF-8")
+    cd = ccall((:libiconv_open, libiconv), Ptr{Nothing}, (Cstring, Cstring), enc, "UTF-8")
     if cd == Ptr{Nothing}(-1)
         return false
     else