cram/cram_external.c: fix external htscodecs include

[emollier]

This patch fixes a duplicate file extension, causing otherwise the following build failure when building htslib against an externally built htscodecs:

gcc -g -O2 -ffile-prefix-map=/<<PKGBUILDDIR>>=. -fstack-protector-strong -Wformat -Werror=format-security -ffat-lto-objects -ffat-lto-objects  -I. -I. -DSAMTOOLS=1 -Wdate-time -D_FORTIFY_SOURCE=2 -c -o cram/cram_external.o cram/cram_external.c
cram/cram_external.c:46:10: fatal error: htscodecs/rANS_static4x16.h.h: No such file or directory
   46 | #include <htscodecs/rANS_static4x16.h.h>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[daviesrob]

Thanks for the fix.