pinkindexer: remove erroneous sys/errno.h

While compiling CrystFEL using musl, this error message popped up:

In file included from ../libcrystfel/src/indexers/pinkindexer.c:36:
x86_64-unknown-linux-musl/sys-include/sys/errno.h:1:2:
  warning: #warning redirecting incorrect #include <sys/errno.h> to <errno.h> [-Wcpp]
    1 | #warning redirecting incorrect #include <sys/errno.h> to <errno.h>
      |  ^~~~~~~

So I removed sys/errno.h completely (since I couldn't figure out where it was used in the file anyways), and the result still compiled.