measunit_extra.cpp: Fix error: 'abs' ambiguous

[Dave-Allured]

measunit_extra.cpp, lines 687-689:
This code is used to check an input string for a non-integer value.
With some older compilers, this generates "error: call to 'abs' is ambiguous".

        uint64_t int_result = static_cast<uint64_t>(double_result);
        const double kTolerance = 1e-9;
        if (abs(double_result - int_result) > kTolerance) {

This PR will avoid the "abs" error by replacing the handcrafted fractional test, with appropriate use of "modf".

I am not experienced with C++. Someone please check this work, thank you.

Downstream issue with complete build log: https://trac.macports.org/ticket/73705
Platform was: macOS 10.14.6 (darwin/18.7.0) arch i386
Compiler was: Apple LLVM version 10.0.1 (clang-1001.0.46.4)
Dialect flag was: -std=gnu++11

TODO: Please read the following on ICU Contributing, and then delete these instructions.
-- Did not read. If this PR is not tolerable, let me know and I will fix.

• For general info on contributing: https://github.com/unicode-org/icu/blob/main/CONTRIBUTING.md
  -- Did not read.

• We require each pull request to be associated with a Jira issue.
  -- Did not file a Jira issue.

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