[BUILD] Add missing CMake keyword for target_link_libraries

[ThomsonTan]

Since the same target was linked with PRIVATE keyboard (see below), it should have explicit keyword for all the dependencies. Or there will be CMake error like the following.

  target_link_libraries(
    example_otlp_instrumented_http PRIVATE common_metrics_foo_library

CMake Error at examples/otlp/CMakeLists.txt:117 (target_link_libraries):
The keyword signature for target_link_libraries has already been used with
the target "example_otlp_instrumented_http". All uses of
target_link_libraries with a target must be either all-keyword or
all-plain.

Changes

Please provide a brief description of the changes here.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[netlify]

✅ Deploy Preview for opentelemetry-cpp-api-docs canceled.

Name	Link
🔨 Latest commit	c70f14a
🔍 Latest deploy log	https://app.netlify.com/projects/opentelemetry-cpp-api-docs/deploys/68308873d550180008246300

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.88%. Comparing base (c2ea734) to head (c70f14a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3442      +/-   ##
==========================================
+ Coverage   89.86%   89.88%   +0.02%     
==========================================
  Files         212      212              
  Lines        6941     6941              
==========================================
+ Hits         6237     6238       +1     
+ Misses        704      703       -1     

see 1 file with indirect coverage changes

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