Include sys/syscall.h to appease g++-16

[eddelbuettel]

When compiling under g++-16 (here on Debian-based container) we now fail for a lack of sys/syscall.h which is likely due to another round of tighter include management. The fail is from my R package rspdlite, it treats spdlite in a fairly vanilla way (not using cmake because of the way R builds packages):

g++-16 -std=gnu++20 -I"/usr/share/R/include" -DNDEBUG -I../inst/include -I'/usr/lib/R/site-library/Rcpp/include'     -fpic  -g -O2 -ffile-prefix-map=/build/reproducible-path/r-base-4.6.0=. -fstack-protector-strong -fstack-clash-protection -Wformat -Werror=format-security -fcf-protection -Wdate-time -D_FORTIFY_SOURCE=2   -c formatter.cpp -o formatter.o
In file included from ../inst/include/spdlite/logger.h:19,
                 from ../inst/include/rspdlite:8,
                 from formatter.cpp:5:
../inst/include/spdlite/formatter.h: In lambda function:
../inst/include/spdlite/formatter.h:33:52: error: ‘SYS_gettid’ was not declared in this scope; did you mean ‘gettid’?
   33 |         raw = static_cast<std::uint64_t>(::syscall(SYS_gettid));
      |                                                    ^~~~~~~~~~
      |                                                    gettid
make: *** [/usr/lib/R/etc/Makeconf:199: formatter.o] Error 1
ERROR: compilation failed for package ‘rspdlite’

One we add the line this PR adds conditional on being on Linux, things work:

g++-16 -std=gnu++20 -I"/usr/share/R/include" -DNDEBUG -I../inst/include -I'/usr/lib/R/site-library/Rcpp/include'     -fpic  -g -O2 -ffile-prefix-map=/build/reproducible-path/r-base-4.6.0=. -fstack-protector-strong -fstack-clash-protection -Wformat -Werror=format-security -fcf-protection -Wdate-time -D_FORTIFY_SOURCE=2   -c formatter.cpp -o formatter.o
g++-16 -std=gnu++20 -shared -L/usr/lib/R/lib -Wl,-z,relro -o rspdlite.so RcppExports.o formatter.o -L/usr/lib/R/lib -lR
installing to /usr/local/lib/R/site-library/00LOCK-work/00new/rspdlite/libs

I can detail this more or provide a standalone reproducible example if you prefer.