signed/unsigned compare in btree.h

Describe the bug

Since the enum containing kNodeValues was changed to be explicitly uint32_t, there is now a signed/unsigned comparison:

external/abseil/absl/container/internal/btree.h:2439:19: error: comparison of integer expressions of different signedness: 'int' and 'unsigned int' [-Werror=sign-compare] 2439 | if (max_count < kNodeValues) { | ~~~~~~~~~~^~~~~~~~~~~~~

Steps to reproduce the bug

Compile revision c6b3f2cf583d8fec124f9d70a172b513363506fe with sign comparison warnings turned on

What version of Abseil are you using? HEAD (c6b3f2cf583d8fec124f9d70a172b513363506fe and later)

What operating system and version are you using Linux (RHEL 6)

What compiler and version are you using? gcc 9.1.0 clang 8.0.1 clang 9.0.0

What build system are you using? cmake 3.15.0

Additional context