[libc][CMake] Set library type of libcMPFRWrapper to STATIC (#99527)

Summary:
Fixes linker errors due to hidden symbols when running CMake with
-DBUILD_SHARED_LIBS=ON.

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Differential Revision: https://phabricator.intern.facebook.com/D60251312

Author: overmighty

libc/utils/MPFRWrapper/CMakeLists.txt

@@ -1,5 +1,5 @@
 if(LIBC_TESTS_CAN_USE_MPFR)
-  add_library(libcMPFRWrapper
+  add_library(libcMPFRWrapper STATIC
     MPFRUtils.cpp
     MPFRUtils.h
     mpfr_inc.h