[gdb/testsuite] Fix gdb.opt/inline-small-func.exp with clang

[gdb/testsuite] Fix gdb.opt/inline-small-func.exp with clang

When running test-case gdb.opt/inline-small-func.exp with clang 12.0.1, I run
into:
...
gdb compile failed, /usr/bin/ld: inline-small-func0.o: in function `main':
inline-small-func.c:21: undefined reference to `callee'
clang-12.0: error: linker command failed with exit code 1 \
  (use -v to see invocation)
UNTESTED: gdb.opt/inline-small-func.exp: failed to prepare
...

Fix this by using __attribute__((always_inline)).

Tested on x86_64-linux.