When building with a C++ compiler, remove the 'register' keyword to silent a warning
When building with a C++ compiler, remove the 'register' keyword to silent a warning

For example, with clang:

lz4.c:XXX:36: error: 'register' storage class specifier is deprecated and incompatible with C++17 [-Werror,-Wdeprecated-register]
static unsigned LZ4_NbCommonBytes (register reg_t val)
                                   ^~~~~~~~~

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