add #include for memcpy to fix compile error on clang 12

[mlaux]

Let's #include string.h so we get memcpy. Fixes this diagnostic when compiling with clang 12:

../common/lzma/MtDec.c:449:19: error: implicitly declaring library function
      'memcpy' with type 'void *(void *, const void *, unsigned long)'
      [-Werror,-Wimplicit-function-declaration]

[rickgaiser]

Thanks!