Export only those symbols that are part of public API

Export only those symbols that are part of public API

Specify -fvisibility=hidden parameter when linking the shared library
using -fPIC, assuming that gcc >= 4 is used.

This change results to unexporting of the following 42 functions:

LZ4F_getErrorCode
LZ4_XXH32
LZ4_XXH32_canonicalFromHash
LZ4_XXH32_copyState
LZ4_XXH32_createState
LZ4_XXH32_digest
LZ4_XXH32_freeState
LZ4_XXH32_hashFromCanonical
LZ4_XXH32_reset
LZ4_XXH32_update
LZ4_XXH64
LZ4_XXH64_canonicalFromHash
LZ4_XXH64_copyState
LZ4_XXH64_createState
LZ4_XXH64_digest
LZ4_XXH64_freeState
LZ4_XXH64_hashFromCanonical
LZ4_XXH64_reset
LZ4_XXH64_update
LZ4_XXH_versionNumber
LZ4_compressHC
LZ4_compressHC2
LZ4_compressHC2_continue
LZ4_compressHC2_limitedOutput
LZ4_compressHC2_limitedOutput_continue
LZ4_compressHC2_limitedOutput_withStateHC
LZ4_compressHC2_withStateHC
LZ4_compressHC_continue
LZ4_compressHC_limitedOutput
LZ4_compressHC_limitedOutput_continue
LZ4_compressHC_limitedOutput_withStateHC
LZ4_compressHC_withStateHC
LZ4_compress_fast_force
LZ4_compress_forceExtDict
LZ4_createHC
LZ4_decompress_safe_forceExtDict
LZ4_freeHC
LZ4_resetStreamStateHC
LZ4_sizeofStreamStateHC
LZ4_slideInputBufferHC
LZ4_uncompress
LZ4_uncompress_unknownOutputSize