converters/pdf2djvu: fix build on GCC architectures

[pkubaj]

Summary

When compiling with GCC, as used on GCC architectures, it errors with:
sys-uuid.cc:96:12: error: 'strlen' was not declared in this scope

assert(strlen(s) == 36U);
       ^~~~~~

sys-uuid.cc:96:12: note: 'strlen' is defined in header '<cstring>'; did you forget to '#include <cstring>'?

[tcberner]

Lgtm